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Old Faithful Geyser in Eruption, Yellowstone Park, Wyoming. 

[Frontispiece.) 



ECLECTIC 



PHYSICAL GEOGRAPHY 



RUSSELL HINMAN 





1%U 



.^\ 



NEW-YORK •:• CINCINNATE-SfT CHICAGO 

AMERICAN BOOK COMPANY 



\ 



The Eclectic Geographies. 

Tivo- Book Series. 



ELEMENTARY GEOGRAPHY $0.55 
COMPLETE GEOGRAPHY $1.20 

THESE GEOGRAPHIES ARE UNRIVALLED in 
simplicity and precision of text, and scientific accuracy 
of maps. Descriptive price-list on application. 



Copyright, 1S88, by Van Antwerp, Bragg & Co. 
Copyright, 1897, by American Book Company. 



EC. PHYS. GEOG. 
E-P 24 



PREFACE 



The aim of this book is to indicate briefly what we know or sur- 
mise concerning the proximate causes of the more common and 
familiar phenomena observed at the earth's surface. Even thus re- 
stricted, the field of inquiry encroaches to a greater or less extent 
upon the domains of all the branches of science. 

Since the study of Physical Geography precedes that of the 
sciences in most of our schools, it has been thought advisable to 
present, in the form of an introductory chapter, a condensed state- 
ment of the more important and fundamental scientific conceptions 
regarding the properties and phenomena of matter and energy, 
such as inertia, gravitation, cohesion, affinity, and heat, light, mag- 
netism, and electricity. This chapter may be studied or simply read, 
at the discretion of the teacher. 

The order of treatment of the different parts of the subject proper 
is that which seems most natural and rational. After describing 
the relations of the planet to the solar system, its movements and 
their effects, the atmosphere is at once considered, not only be- 
cause it forms the true outer layer and envelope of the earth, but 
because its action is the proximate cause both of all details in the 
relief of the land, and of the more conspicuous phenomena of 
the sea. The sea is next discussed, since it forms an intermediate 
layer between the atmosphere and three fourths of the earth's solid 
surface, and since the peculiarities in the relative position, compo- 
sition, and relief of the land masses can be appreciated only after 
some acquaintance with the depth and character of the bottom in 
different regions of the sea. In the treatment of the land, which 
then follows, the methods by which its surface contour is constantly 
modified by atmospheric agencies are explained at greater length 
than is usual in current text-books, while the influence of subter- 
ranean agencies in changing the elevation of the land is carefully 
considered. Climate, being the average local condition of the atmos- 
phere, as determined largely by the peculiarities of the surface upon 

(iii) 



IV PREFACE. 

which it rests, is appropriately treated after that surface has been 
discussed, and fittingly precedes the concluding chapters on life. 
These chapters embrace a brief description of the more conspicuous 
phenomena of the organic world. The close dependence of plants 
and animals upon their inorganic surroundings and upon each other 
is pointed out, as well as the remarkable series of facts which is held 
by many scientists to indicate that all organisms are of kin. 

In a study of this kind it is well to remember that, with all the 
scientific knowledge of the nineteenth century, we are still profoundly 
ignorant of the ultimate causes of things, while our ideas of prox- 
imate causes are constantly being revised and changed as our 
acquaintance with nature increases. The broadest scientific gener- 
alizations of one generation are apt to be swept away or greatly 
modified by the next, and our descendants will doubtless regard 
the science of to-day much as we regard that of the ancients. Yet, 
notwithstanding its crudities and absurdities, ancient alchemy grad- 
ually developed into modern chemistry, which has been of inesti- 
mable value to man ; and in the same way, the more perfect 
knowledge of the future is to be acquired only through familiarity 
with the imperfect theories of to-day. 

The maps in the book have been carefully prepared in various 
projections, each adapted to portray most accurately the special 
feature under consideration, and, with the cuts and diagrams, are 
inserted to illustrate, and not simply to beautify the text. 

The acknowledgments of the author are due to the United States 
Geological Survey, Coast and Geodetic Survey, Signal Service, and 
Hydrographic Bureau for maps and information courteously sup- 
plied. He also takes this opportunity to acknowledge gratefully the 
valuable assistance rendered in revising the proof-sheets of the 
introductory chapter, by Prof. T. H. Norton, of the University of 
Cincinnati ; of the chapters on the atmosphere and climate, by 
Prof. Cleveland Abbe, of the U. S. Signal Service, and Prof. W. M. 
Davis, of Harvard College ; and of the chapters on the land by 
Capt. C. E. Dutton of the U. S. Geological Survey. 



CONTENTS. 



Introduction. — Some General Laws of Nature 

Part I. — The Earth as a Planet. 



Chapter I. — The Solar System . 

" II. — Movements of the Earth 



PAGE 

7 



35 
43 



Part II. — The Atmosphere. 

Chapter III. — Composition, Weight, and Heat . . -55 

" IV. — Moisture of the Atmosphere .... 66 

" V. — Movements of the Atmosphere ... 78 

VI, — " " " " (Continued) . 90 

" VII. — Luminous Phenomena 100 

Part III.— The Sea. 

Chapter VIII. — Depth, Composition, and Temperature . . 109 

IX. — Waves and Tides . . . . . . 122 

X. — Currents and Deposits 135 



Part IV. — The Land. 



Chapter XI. — Divisions of the Land 
XII. — Surface of the Land 
XIII. — Structure of the Land 
XIV. — Springs 
XV. — Streams 
XVI.— Work of Streams 
XVII. — Glaciers and Lakes 



149 
161 
180 

195 
205 
216 
231 



(v) 



VI 



CONTENTS. 



Chapter XVIII. — Mountain Structure and Land Sculpture 
XIX. — Earthquakes ..... 
XX. — Volcanoes 



PAGE 
. 248 
. 264 
. 278 



Part V. — Weather and Climate. 
Chapter XXI. — Weather and Climate . 



293 



Part VI. — Life. 

Chapter XXII. — The Various Forms of Life 
XXIIL— Distribution of Life 
XXIV.— Man .... 

Index 



3L3 

328 

35° 
373 



List of Maps and Charts. 



Abnormal Temperatures of 




Isothermals, United States . 


309 


the World 


305 


Lake Region United States . 


239 


Biological Regions 


334 


Magnetic Meridians, North- 




Continental Plateau . 


152 


ern Hemisphere 


3i 


Currents and Floating Ice 




Malay Archipelago 


156 


of the Sea 


138 


Map Projections 


53-4 


Delta of Mississippi and Nile 


228 


Rain-fall on Land Surface 




Depths of the Sea . .11 2-3 


of the World . 


76 


Drainage Basin of Missis- 




Rain-fall of United States . 


3°7 


sippi River 


207 


Range of Annual Tempera- 




Drainage Basin of the Oceans 


209 


ture of World 


300 


Earthquakes in the U. S. . 


267 


Storm Frequency and Paths 


9 1 


Glaciers and Glaciated Re- 




Surface Africa and Australia 


175 


gions .... 


238 


Surface of Euro-Asia . 


173 


Isobars and Winds Northern 




Surface of North and South 




Hemisphere 


87 


America 


167 


Isobars and Winds Southern 




Tornado Frequency in the 




Hemisphere 


85 


United States . 


98 


Isothermals, Northern Hemi- 




Vegetation regions of World 


33 2 


sphere .... 


63 


Volcanic regions of World . 


289 


Isothermals, Southern Hemi- 




Weather Chart of eastern 




sphere .... 


65 


United States . 


296 



INTRODUCTION. 



SOME GENERAL LAWS OF NATURE. 

Show vie thy ways, O Lord, teach me thy paths. — Psalm xxv: 4. 

Laws of Nature. — Nothing in nature is permanent ; 
every thing is constantly changing. Day changes into night, 
fair weather into foul, plants and animals die and decay, and 
even the solid rocks gradually wear away into soil or sand. 
These changes do not occur by chance, but each is the 
result of some definite cause, and, under similar circum- 
stances, precisely the same effects are produced by the 
same causes. The invariable relations between causes and 
resulting effects constitute the laws of nature. 

Physical Geography seeks to trace the operation of 
the laws of nature upon the earth ; upon the air, the 
water, and the land ; upon plants, animals, and even upon 
man. 

Matter is the general name given to the material of 
which any body, such as rock, water, or air, is composed. 

Kinds of Matter. — The number of substances in the 

world is almost infinite ; yet most of them are compound 

substances, and can be broken up into a comparatively 

few distinct kinds of matter, from which no other kind can 

be obtained. These are, therefore, called simple substances, 

or elements. 

(7) 



PHYSICAL GEOGRAPHY. 



Constitution of Matter. — All matter is conceived to 
be built up of minute particles, called molecules. A mole- 
cule is the smallest fragment of any substance that can 
exist by itself. A molecule of a compound substance 
breaks into the simple substances of which the compound is 
composed ; and a molecule of a simple substance breaks 
into still smaller fragments, called atoms, which are too small 
to exist by themselves, but large enough to unite with 
other atoms of the same or different kinds of matter to 
form a simple or a compound molecule. 

Molecules are too small to be visible even with the aid of the 
most powerful microscope. Some idea of their extreme smallness 
may be gathered from Sir William Thomson's estimate. He says 
that if a drop of water were magnified to the size of the earth, its 
molecules, so magnified, would be about as large as base-balls. 

Common Properties of Matter. — All matter of what- 
ever kind is indestructible, impenetrable, and possesses 
inertia. By virtue of the first quality, 
it is absolutely impossible to destroy 
a single atom or molecule in nature. 
A substance may disappear, but new 
substances are always formed of its 
constituent atoms. By virtue of the 
second quality, two particles can not 
occupy the same space at the same 
time. A nail driven into a board 
does not penetrate the molecules of 
the wood : it simply forces them aside. 

Inertia. — All matter resists being set in motion, and, 
when moving, resists any change in the rate or direction 
of its motion. Hence, no body can either start into mo- 
tion or stop when moving unless something outside of itself 
pulls or pushes it powerfully enough to overcome this re- 
sistance. This resistance of matter is called its inertia. 




Fig. i. 



SOME GENERAL LAWS OF NATURE. 9 

The inertia of bodies increases with the amount of matter they 
contain. Thus, if two balls of iron, a large one and a small one, be 
suspended by long cords, the large one will require a more powerful 
pull or push to start it to swinging or to stop it than the small one. 
If a ball of cork, of the same size as one of the iron balls be sus- 
pended, it will require a less powerful pull or push to start or stop it 
than the iron ball. The iron ball, therefore, though of the same 
size, contains a greater quantity, or mass, of matter, and possesses 
greater inertia than the cork ball. 

Forces of Nature. — A pull or a push of any kind or 
strength always tends to overcome inertia, and is called a 
force. There are three great natural forces by whose 
varying action the few elemental substances are gathered 
and held together into the infinite variety of groups or 
bodies of nature. These forces are Gravitation, Cohesion, 
and Chemical Affinity. 

Gravitation is a force which is constantly acting upon 
all matter in nature. By virtue of this force, each mole- 
cule of matter tends to attract, or pull toward itself, every 
other molecule, however distant. Since each molecule 
exercises this attraction upon others, a body composed of 
a great number of molecules, or of a large mass of matter, > 
exercises a stronger attraction than bodies of fewer mole- 
cules or less mass. The mass of the whole earth is so 
enormous that its attraction quite overpowers that of de- 
tached bodies near its surface ; these, therefore, if unsup- 
ported, yield to the attraction of the greater mass, and 
move or fall toward the earth. If the body is sup- 
ported, the attraction of the earth causes it to push or 
press against its support. This pressure is called the 
weight of the body. 

Some bodies do not fall, but ascend — as smoke or a balloon in 
the air, or a cork or oil under water. This is not because the earth 
does not attract them, but because an equal bulk of the air or water 
immediately above the body contains a greater mass of matter, and 
is, therefore, more strongly attracted by the earth than the body 



10 



PHYSICAL GEOGRAPHY. 



itself. The body and the greater mass of air or water above it con- 
sequently exchange places- — the greater mass sinking, and forcing 
the smaller mass to rise.. When two bodies are weighed in the same 
place and under similar conditions, the heavier always contains the 
greater mass of matter even if it is much the smaller body. Thus, 
i cubic foot of rock, 2 cubic feet of water, 8 cubic feet of cork, and 
1 ,600 cubic feet of air have about the same weights and the same 
inertia, and, consequently, are equal to each other in mass, though 
the bulk of the cork is four times that of the water, and eight times 
that of the rock. If the bulks were equal, the rock would weigh 
twice as much as the water, the cork one fourth as much, and the air 
but one eight hundredth as much. The specific gravity of a sub- 
stance is such a comparison of its weight with that of an equal bulk 
of some other substance, usually water, taken as a standard. Thus, 
the weight of water being called one, the specific gravity of rock is 
two, of cork one fourth, of air one eight hundredth. 




Fig. 2. 



Distance. — Although the attraction of gravitation acts 
between bodies, however far apart they may be, the power 
or intensity of this force decreases very rapidly as the dis- 
tance between the bodies increases. Thus, gravity acting 
from B is distributed over four times the space at 2D, and 
nine times the space at 3Z? that it is at \D ; hence, if the 
distance between two bodies is doubled or trebled, the 
mutual attraction of gravitation which they exert on each 
other is decreased to one fourth and one ninth respect- 
ively ; in other words, the intensity of gravitation varies in- 
versely as the square of the distance between the bodies. This 
law applies to sound and to radiant heat and light as well 
as to gravitation. 



SOME GENERAL LAWS OF NATURE. I I 

The effects of Gravitation are almost infinite in num- 
ber, and many of them are familiar to every one. In 
general, gravitation gives weight to all substances in nature — 
even to such light and invisible substances as air — and 
therefore this force is one of the causes of all phenomena 
in which the weight of bodies plays a part. The rising 
of vapor through the heavier air, and the falling of rain 
through the lighter air ; the moving of heavy air, as wind, 
to a region where the air is lighter; and the downward 
flow of streams over the sloping surface of the land, — all 
these are caused by the attraction of the enormous mass of 
the earth upon the relatively insignificant masses of matter 
near its surface. 

The effect of the earth's attraction, however, is not confined to the 
neighborhood of its surface. The nearest heavenly body, the 
moon, is much smaller than the earth ; it contains but one eightieth 
as much matter. Though 240,000 miles distant, the attraction of the 
larger earth pulls this body constantly from the straight course which 
its inertia influences it to follow, and causes the moon's path or orbit 
to become nearly circular around the earth. The moon's attraction 
upon the larger earth is imperceptible on the solid land, but it heaps 
up the waters of the sea to form the tides. But the attraction of 
gravitation extends to all distances. The sun is a body whose mass 
is 300,000 times as great as that of the earth and moon together. 
The attraction of this vast mass, exerted through a distance of 
92,000,000 miles, overcomes the earth's inertia, and causes the earth 
and the other planets of the solar system to move around the sun in 
nearly circular orbits — just as the earth influences the movement of 
the moon. The effect of the sun's attraction upon the earth is seen 
in the regular recurrence of the seasons, and the regular variation in 
the length of the day and night. Each fixed star is, probably, like 
our sun, the center of a system of planets. Each of these systems, 
as a whole, has, like the solar system, a motion of its own through 
limitless space, which is modified and regulated according to the 
mass and distance of the various systems by this universal force 
of gravitation. 

Cohesion, like gravitation, is an attractive force which 
may act on every molecule in nature. But it is unlike gravi- 



12 PHYSICAL GEOGRAPHY. 

tation in three important particulars: (i) it acts only be- 
tween individual molecules, and not between masses ; (2) 
it acts only between molecules of the same kind of matter ; 
and (3) it acts only when the molecules are exceedingly 
close together, — so close that they seem to be in absolute 
contact. 

It is the force of gravity acting between the mass of the earth and 
the mass of a stone which causes the stone to fall to the earth ; but 
it is the force of cohesion acting between the millions of individual 
stone molecules which causes them to stick together and form the 
solid mass of stone. A man lifting a bucket from a well, or carry- 
ing a heavy basket, is thus overcoming the force of gravity ; while a 
woodman chopping down a tree, or a machinist filing a piece of 
iron, is struggling against the force of cohesion. 

State of Aggregation. — It is believed- that between the 
molecules of all bodies two antagonistic forces are con- 
stantly struggling for mastery : the attractive force of co- 
hesion, and a repulsive force, which results in what we call 
"heat." When the attractive force is the stronger, the 
molecules cohere (stick together) and form a solid; when 
the two forces are about equal, the molecules move about, 
over, and beside each other, and form a liquid ; when the 
repulsive force is the stronger, the molecules fly farther 
apart, and form a gas. 

It thus depends simply upon the relative intensities of cohesion 
and heat between the molecules of any substance, whether that sub- 
stance takes the form of a solid, a liquid, or a gas. Ice, water, and 
steam, for example, may be produced from the same molecules by 
simply making them colder or hotter. It is believed that there is no 
substance which may not exist in any one of the three states of ag- 
gregation, — a solid, a liquid, or a gas. 

Crystallization is a peculiar effect of cohesion fre- 
quently seen in rock, cast iron, snow, and very many 
other solids. When a substance passes slowly and quietly 
from a liquid to a solid state, the molecules generally 
arrange themselves in a peculiar manner, assuming certain 



SOME GENERAL LAWS OF NATURE. 



13 



definite geometrical shapes. These shapes vary in differ- 
ent substances, but remain constant in substances of the 
same kind. 

If water, for instance, be examined when solidifying, or freezing, 
delicate needles of ice will be observed shooting out over the sur- 
face and forming six-pointed stars or little six-sided figures. These 
are ice crystals, and, if observed in freezing water in any part of 
the world, the ice-needles are always found to form angles of just 
6o° with each other. If a solution of table salt be allowed to solidify, 
the salt takes the form of little cubes with exquisitely smooth sides, 
and clean, square angles. These are salt crystals. In general, every 
substance forms crystals, which are always the same in the same 




Fig. 3. — 1. Quartz. 2. Gypsum. 



Salt. 4. Calcspar. 5. Feldspar. 



substance, but different in shape from the crystals of other sub- 
stances. Crystallization is explained by supposing that the cohesive 
force of molecules is not exerted equally on all sides, but is stronger 
in certain definite directions, and that there are differences in this 
respect between the molecules of different kinds of matter. 

The power of Cohesion is, of course, greatest in 
solids, but it varies in different substances. The cohesive 
attraction is so strong that it requires a pull of 

100,000 to 170,000 lbs. to break a steel bar 1 inch square. 

50,000 to 100,000 lbs. " " " iron " " " 

4,000 to 20,000 lbs. " " " wooden " " " 

500 to 1,000 lbs. " " "stone " " " 

The change in the arrangement of water molecules, 
under the force of cohesion causes the bursting of water- 
pipes, when the water in them crystallizes, ox freezes. 

The molecules of water and of some other substances occupy a 
greater space when the absence of heat allows cohesion to arrange 



14 PHYSICAL GEOGRAPHY. 

them in the form of crystals, than when, heat being present, it over- 
comes cohesion and arranges the molecules so that they slide over 
each other in the form of a liquid. Hence, the contents of a water- 
pipe expands as it changes from a liquid to a crystallized form. It 
expands with a force which, at the ordinary temperature of freezing 
water, produces an outward pressure of more than 2,000 pounds on 
each square inch of the interior of the pipe, and this pressure in- 
creases rapidly as the temperature falls. Few pipes can withstand 
this pressure, and, consequently, they burst. If the pipe is strong 
enough, however, to prevent the expansion, the water can not freeze, 
but remains liquid. 

Adhesion is a force similar to cohesion, except that 
while cohesion acts only between molecules of the same 
kind of matter, adhesion acts only between molecules of 
different kinds. 

No body can become wet when plunged into water, if the adhe- 
sion between the molecules of the water and the body is weaker 
than the cohesion between the molecules of the water. Grease 
is such a substance, and, therefore, does not become wet. 

Capillary Attraction is an instance of adhesion, and 
is so named from the Latin word capillus, a hair. When 
one end of a fine, hair-like tube is plunged into any liquid 
that will adhere to the material of which the tube is made, 
the attraction of adhesion causes the liquid to rise in the 
tube a short distance above the level of the liquid outside 
the tube. The finer the bore of the tube, the higher will 
the liquid rise in it. 

If the corner of a towel be allowed to remain for a short time in 
water, the towel, for some distance above the surface of the water 
will be found to be wet, the minute spaces between the strands of 
the threads having acted as so many tubes in which the water rises 
by capillary attraction. 

Capillary attraction plays a very important part in nature ; it 
enables the soil and rocks to retain water, it forms one of the 
means by which plants are supplied with their liquid food, and it is 
called into use in distributing some of the animal juices through- 
out the body. 



SOME GENERAL LAWS OF NATURE. 1 5 

Chemical Affinity, like gravitation, is an attractive 
force, and, like cohesion, acts only at imperceptible dis- 
tances. But it is an atomic force ; it acts, primarily, only 
between atoms, and when these atoms are of different 
kinds of matter, a substance is produced entirely different 
from either of the atoms. 

The difference between affinity and cohesion is thus very appar- 
ent ; cohesion increases the mass of a substance by adding together 
many minute particles of the same substance ; affinity produces a 
new substance by combining still more minute particles of totally 
dissimilar substances. 

Elements. — It has been said that an element or simple 
substance is composed of but a single kind of matter ; 
that is, if an element could be broken up into its atoms, 
all the atoms would be exactly alike in all particu- 
lars. There are about seventy known elements ; of these, 
fifty-five are called metals, such as aluminium, calcium, 
potassium, iron, copper, gold, etc. The remaining ele- 
ments are called metalloids ; some are solids at ordinary 
temperatures, such as silicon, carbon (charcoal), p/iosp/iorus, 
and sulphur; one is a liquid, — bromine; and others are 
gases at ordinary temperatures, such as oxygen, hydrogen, 
and nitrogen. 

Compounds. — Every substance in nature is composed 
of one or more elements in a free state, or is a chemical 
combination of two or more elements. Such a combina- 
tion produces a compound substance. 

Air is composed chiefly of oxygen and nitrogen mixed together 
in the free states. Water is a chemical compound of the two ele- 
ments, oxygen and hydrogen ; quartz, ox flint, of oxygen and silicon; 
limestone, of oxygen, carbon, and calcium ; bread, meat, and most 
foods, of oxygen, hydrogen, nitrogen, and carbon, together with 
minute quantities of various other elements. 

Oxygen is far the most abundant element on the earth. 
At ordinary temperatures it is a gas — colorless, tasteless, 

P. G.— 2. 



1 6 PHYSICAL GEOGRAPHY. 

and odorless, and a trifle heavier than air. Oxygen is 
essential to all life, both animal and vegetable. 

Oxygen forms about \ by weight of all rocks of the earth. 
" f " " " " water. 

II (( II 1 II II l< II _• 

z air - 

" f " " " " vegetable matter. 
" § " " " " animal matter. 

Oxidation and Combustion. — The affinity of oxygen 
for most of the substances in nature is so strong that it is 
constantly combining with them when they are exposed 
to the air with its large quantity of free oxygen. The 
process is called "rusting," "rotting," or "burning" in 
different instances, all of which are covered by the general 
term oxidation. Oxidation always produces heat, as is 
the case with most chemical combinations. When the 
heat is great enough to produce light, the process is 
called combustion. 

A familiar instance of oxidation is the "rusting" of iron in the 
air The oxygen of the air unites with the surface iron to form the 
new substance, oxide of iron. The burning of fuel is really the 
same process, only the chemical affinity between the carbon and 
hydrogen of which most fuel is composed, and the oxygen of the 
air, is much greater than that between iron and oxygen ; the heat 
produced by their union is great enough to produce light, and the 
process is consequently called combustion. 

Decomposition is the breaking up of a compound into 
its elements or into simpler compounds. The affinity be- 
tween different elements varies in strength. Whenever 
elements having a stronger affinity for each other than 
for those with which they happen to be combined, are 
brought by any means sufficiently close together for affinity 
to act, the weak combinations break up, and the ele- 
ments, liberated by the decomposition, recombine accord- 
ing to the strength of their mutual affinities to form other 
substances, which, under similar circumstances, decompose 



SOME GENERAL LAWS OF NATURE. 1 7 

and recombine into still other compounds. In this way, 
the atoms and molecules are kept in circulation. 

Gunpowder is nothing but certain weak combinations containing 
elements that have a strong affinity for each other, artificially placed 
so close together that a slight heat causes the weak combinations to 
decompose and leave certain of their ingredients free to unite in a 
strong combination. 

Kinetic Energy. — One or more of these three great 
attractive forces — gravitation, cohesion, or affinity — is 
thought to be, directly or indirectly, the cause of all mo- 
tion in the universe ; and every one of the innumerable 
changes that are constantly taking place in matter around 
us, is thought to be the result of some kind of motion of 
that matter, — either a visible motion of its mass as a 
whole, or an invisible motion of its molecules or atoms. 
Matter in motion always imparts some kind of motion to 
other matter with which it comes in contact. This im- 
parting of motion is called doing work. Matter in motion 
is, therefore, said to have the power to do work, or to 
possess kinetic {active) energy. 

The quantity of work a body in motion is capable of doing, — or 
the amount of kinetic energy it possesses, — depends more upon the 
rapidity of its motion than upon its mass ; for, while doubling the 
mass only doubles the energy, doubling the speed increases the 
energy four times ; thus, energy increases zvith the mass of a body 
but with the square of its vetocity. That is, a hundred-pound cannon- 
ball, moving with a certain speed, possesses no more energy than a 
one-pound ball moving ten times as fast. 

Potential Energy. — It often happens that the expendi- 
ture of kinetic energy upon a body places it in such a 
position that, though not in motion, it would move and 
do work if unrestrained. A body in such a position is 
said to possess potential {possible) energy. A bent bow, a 
hoisted weight, or a wound-up watch spring possesses po- 
tential energy. 



15 PHYSICAL GEOGRAPHY. 

Potential energy is thus simply stored-up kinetic energy, since it 
exists only in bodies on which kinetic energy has been expended. 
Upon the removal of restraint, the body immediately moves, and the 
potential energy is converted back again into exactly the amount of 
kinetic energy which was expended in its production. 

Conservation of Energy means that the total amount 
of energy in the universe is an unchangeable quantity. 
Energy, therefore, can not be created or destroyed, but it 
is constantly eluding observation by changing its form, or 
by entering and leaving different masses of matter. It is 
frequently apparently destroyed either (i) when it changes 
from a kinetic to a potential form, — as when the kinetic 
energy expended in lifting a heavy weight is changed into 
potential energy of the weight when the latter stops moving 
just before falling ; or (2) when masses in visible motion impart 
a portion or all of their motion and energy to invisible 
molecules, — as when a falling weight strikes the earth, and 
its motion, as a mass, stops. In this case, if the weight 
and the place struck were carefully examined, they would 
be found to have undergone certain changes in conse- 
quence of the blow ; they would be hotter than before, 
and they might have become luminous, or other changes 
might have taken place. 

These changes — heat, light, etc., — are simply the results of molec- 
ular energy arising from the invisible motions imparted to the mole- 
cules by the collision which stopped the visible motion of the weight. 
If the energy causing all the changes that occurred in consequence 
of the blow could be collected, it would be found to equal the amount 
which the weight possessed when it struck the earth, and exactly this 
amount of energy is passed on to other matter by the molecules be- 
fore they regain their former condition. 

Nature of Heat and Light. — Heat and light are the 
results of a certain kind of insensible motion of the mole- 
cules of matter. Therefore, all warm or luminous bodies 
possess kinetic energy by virtue of this motion. A body 



SOME GENERAL LAWS OF NATURE. I 9 

is said to be hot when its molecules possess an exceed- 
ingly rapid, but of course invisible, vibratory motion. 
When the molecular motions increase to a certain rapidity, 
the body becomes luminous, and is said to be "red" hot. 
As the motions become slower, the body ceases to be 
luminous and becomes cooler, but the molecules of no body 
are supposed to be at rest. Hence all bodies, even the 
coldest, are thought to have more or less heat. Whatever 
increases the rapidity of the motions increases the heat of 
the body, and whatever decreases the rapidity causes the 
body to cool. 

Repeated blows of a hammer on a nail, or the friction of one 
body rubbing on another, increases the rapidity of the molecular 
agitation in each, and thus produces heat mechanically. The clash 
of atoms colliding under the attraction of affinity, produces a sim- 
ilar increase, and produces heat chemically. The heat of all " fire " 
is thus produced. 

Transference of Heat. — Unequally heated bodies, 
whether touching each other or not, always tend to acquire 
a uniform temperature, the hotter becoming cooler, and 
the cooler becoming hotter. This is accomplished (1) by 
radiation of energy ; (2) by conduction ; or (3) by con- 
vection. 

Radiation of Energy takes place between unequally 
heated bodies that are not in contact. It is explained by 
supposing that the universe is pervaded by an elastic sub- 
stance called luminiferous ether, so thin that it enters and com- 
pletely fills the invisible interstices between the molecules 
of all substances as easily as water fills the cavities of a 
sponge. The movements of the molecules of all bodies 
tend to produce vibrations in the ether pervading and 
surrounding them, just as any shock sets a bowl of jelly 
in a quiver. The hotter bodies tend to impress quicker 
vibrations on the ether than the cooler ones. A continued 



20 PHYSICAL GEOGRAPHY. 

expenditure of energy (heat) is required to maintain the 
vibrations of the ether, which spread away or radiate in all 
directions ; hence the body that excites the vibrations cools ; 
but if the energy of the vibrations of the ether is expended 
upon, or absorbed by, a substance whose molecules are 
thereby excited to faster motion, the substance is warmed. 
Radiant energy is said to pass from one body to another in 
rays. Some of the rays emitted by very hot bodies are 
perceptible to the nerves of the eye as light ; these rays 
have 392 trillion to 757 trillion vibrations a second. Rays 
of slower or faster vibrations are not perceived by the eye. 
Any ray that is absorbed by a substance, whether visible 
or invisible as light, produces heat. 

It is by radiation of energy that the heat and light of the sun 
reach the earth, and that a person is warmed when standing before 
a fire. Radiant energy travels through ether at the enormous speed 
of 186,000 miles a second. The heat and light of the sun require 
about eight minutes to reach the earth. 

Conduction. — When unequally heated bodies or mol- 
ecules are so close together that they are usually said to be 
in contact — as the molecules are in solids — the more active 
molecules impart some of their motion to the slower mov- 
ing adjacent molecules, and these to their still slower 
neighbors, until a uniform heat and rate of motion is con- 
ducted to the most distant part of the body. In compar- 
ison with radiation, the conduction of heat is exceedingly 
slow ; but dense bodies, such as the metals, conduct heat 
faster than porous bodies, such as snow, earth, rock, etc. 
The former are therefore called good conductors, while the 
latter are called poor or non-conductors. 

Convection. — Liquids and gases are very poor conduc- 
tors, since their molecules can move freely among them- 
selves. Hence, if the upper part of a liquid or gas is 
warmed, a very long time is required to transfer heat to 



SOME GENERAL LAWS OF NATURE. 2 1 

the lower portion; but if heat is applied from below, the 
lower portions generally expand as they grow warmer, and 
thus become lighter than those above. The lower portions 
are therefore forced to rise by the gravity of the heavier 
portions above, and thus convection currents are established, 
which convey the heat throughout the liquid or the gas. 
Reflection, Absorption, and Transmission of Ra- 
diant Energy. — When radiant energy encounters a body, 
it (i) enters the body, or (2) is reflected from its sur- 
face. That which enters may be either transmitted through 
the body and pass out on the opposite side, or it may be 
absorbed (retained in the body). It is only the energy 
that is absorbed that affects the temperature of the body. 
Bodies are called good reflectors, absorbers, or transmit- 
ters of radiant energy according as they reflect, absorb, or 
transmit the greater part of the rays falling on their sur- 
face, though no body is perfect in either respect ; the best 
reflectors absorb some of the energy, the best absorbers 
reflect a portion, and the best transmitters both absorb 
and reflect a little. 

Bodies such as glass, which transmit most of the rapid vibra- 
tions of visible rays, are called transparent. Bodies which transmit 
most of the ether vibrations of either visible or invisible rays, with- 
out being themselves warmed, are called diathermanous. Bodies 
which absorb most of the ether vibrations, and are hence warmed, 
are said to be athermanons. Most bodies are athermanous, and none 
are perfectly diathermanous or transparent. Dry air and rock salt 
are among the most diathermanous substances. The dry, pure air of 
high mountains transmits nearly all the heat of the sun's rays, and is 
itself scarcely warmed by them. On account of this quality of the 
air, a person at the top of a very high mountain might be quite un- 
comfortably warm in the sunshine, and yet water might be freezing 
in the shadow of a rock beside him. Glass, and air rendered slightly 
hazy by fine water globules or dust particles, though diathermanous 
to light rays, are largely athermanous to rays emitted by dark bodies. 
Thus, the window-glass allows the 'sunbeams to enter and warm a 
room, but prevents the dark radiations from the warm interior from 



22 PHYSICAL GEOGRAPHY. 

passing out again. Water, though exceedingly transparent, transmits 
scarcely any dark rays. 

Expansion and Contraction. — When a body grows 
hotter or colder, a change in its size always takes place. 
As a general rule, bodies expand when heated and contract 
when cooled. 

In the ordinary thermometer, or heat measure, the expansion and 
contraction is employed to denote its change of temperature. The 
common thermometer consists of a glass tube of very fine 
bore, terminating in a bulb, which, with part of the tube, is 
filled with some liquid, usually mercury. As the tempera- 
ture of the mercury increases, it expands and mounts higher 
in the tube ; as the temperature decreases, the mercury 
contracts and descends in the tube. The tube is graduated 
by marking the height of the mercury when the bulb is 
held first in freezing and then in boiling water, and marking 
the intervening space into equal divisions called degrees. 
In Fahrenheit's thermometer, which is generally used in 
this country, the freezing point is marked 32 and the boil- 
ing point 212 . When the thermometer is brought into the 
neighborhood of a hot or cold body, the mutual radiation 
between the body and the instrument reduces them to a 
common temperature, which can at once be compared with 



Fi e- 4- that of freezing or boiling water by noting the height of the 
mercury in the graduated tube. 

The power with which substances expand or con- 
tract is practically resistless. The amount of expansion or 
contraction varies in different substances ; thus, for each 
degree of variation in temperature, a mass of air grows 
larger or smaller by about t^qVto > water, 10 ooooo 5 ice > 
TooTToo 5 ir on, yo~U~o ¥To > ano - roc k Dut - looooolF °* ^ s 
bulk or volume. These amounts are so small as to be 
usually imperceptible, but when substances are in large 
quantity, or when the variation in temperature is great, 
the expansion or contraction is very perceptible ; and, be- 
ing resistless, the results are stupendous. 



SOME GENERAL LAWS OF NATURE. 23 

A change of temperature of io° in a mass of air one mile square 
tends to change its length, breadth, and thickness by about thirty-six 
feet. A much greater change of temperature occurs daily through 
millions of cubic miles of the atmosphere. A change of only i° in 
the temperature of a sheet of ice a mile long changes its length 
about \% inches. A contraction of even this small amount accounts 
for the long, fine cracks which open with loud report in the ice of all 
ponds and lakes in cold weather. A part of the earth's rocky crust 
one mile in length would tend to become about 2^ feet longer were 
its temperature increased ioo°. 

Explanation of Expansion and Contraction. — The 
expansion or contraction of a body results from a move- 
ment of its molecules into an arrangement occupying a 
greater or a less space. Cohesion usually resists such 
movements as result in expansion ; hence, part of the 
heat-energy entering a body must counteract the resistance 
of cohesion, and is thus held in a potential or inactive form 
to maintain the altered size of the body, and only the re- 
mainder of the energy is left to cause the change in the 
active motion or quiver of the molecules, which alters the 
temperature. Conversely, when a body cools and contracts, 
it surrenders not only a portion of its active, temperature- 
maintaining energy, but also a portion of its potential, 
size-maintaining energy, which, being relieved of the re- 
sistance of cohesion, becomes active heat-energy as it 
leaves the body. The resistance of cohesion is very 
different in different substances ; hence, the amount of 
heat-energy required to produce the same change of tem- 
perature varies greatly in different substances. 

Water requires a greater amount than almost any other substance. 
Thus, if all the energy liberated in cooling a mass of rock 5 were 
to enter an equal weight of water, it would raise its temperature but 
i°. Conversely, when a given weight of water cools i°, it liberates 
enough energy to raise the temperature of an equal weight of rock 5 . 
Water is therefore said to have a capacity for heat, or a specific 
heat, five times as great as rock. The specific heat of water is about 
four times that of air. 



24 PHYSICAL GEOGRAPHY. 

On account of its great specific heat, water cools or is heated 
more slowly than almost any other substance. Thus, if a pound of 
water, at 50 temperature, is surrounded by a pound of air at 45 
temperature, the water cools and the air becomes warmer until their 
temperatures are the same'; but the water cools only i°, for in doing 
so it liberates enough energy to heat the air 4° ; hence, the resulting 
uniform temperature of air and water is 49 . The bulk of a pound 
of air is about 840 times larger than that of a pound of water ; 
hence, a pond a foot deep, in cooling i°, liberates enough energy to 
heat by 4° the overlying air to a height of 840 feet ; thus, large 
bodies of water have a powerful influence upon the climate in their 
neighborhood. 

Latent Heat. — All bodies require an exceptionally- 
large amount of energy to effect the peculiar re-arrange- 
ment of their molecules when they change from a solid to 
a liquid, or from a liquid into a gaseous state. When a 
solid is heated, its size and temperature increase until it 
begins to melt ; then, though heat is still applied, its tem- 
perature remains unchanged until all of it is melted, the 
entire energy of the heat being required to re-arrange the 
molecules into a liquid form. When this re-arrangement 
of all the molecules is completed, if heat be still applied, 
the size and temperature of the liquid increase until it 
begins to boil or pass into vapor. Here the same thing 
happens ; although heat is applied continuously, all its 
energy is rendered potential by the resistance which cohe- 
sion offers to the alteration of molecular arrangement into 
a gaseous form, and the temperature remains unchanged 
until the liquid has entirely disappeared, after which the 
temperature of the gas begins to increase. 

The energy which thus disappears upon the melting or 
vaporizing of substances, is said to become latent (con- 
cealed) ; for when the substance passes back again into a 
solid or liquid state upon cooling, the latent energy again 
appears as heat, which affects the temperature of sur- 
rounding bodies. 



SOME GENERAL LAWS OF NATURE. 25 

The latent heat of water is greater than that of most other sub- 
stances. It requires as much heat-energy simply to melt a pound of 
ice — without changing its temperature in the least — as is required to 
raise the temperature of 140 pounds of water i°, while the energy 
required to vaporize a pound of water would raise the temperature 
of 980 pounds of water i°. 

Freezing of Water. — Water, iron, and some other 
substances occupy a greater space in the solid than in 
the liquid state, and hence do not expand and contract 
according to the general rule when near their melting points, 
If fresh water be cooled, it contracts regularly till it 
reaches its maximum density at a temperature of 39 , after 
which it slowly expands as it cools, until, in freezing, it 
makes a sudden and great expansion — twelve cubic inches 
of water making about thirteen cubic inches of ice. Ice 
is consequently lighter than an equal bulk of water, and 
hence floats. If the ice be further cooled, it will be 
found to contract regularly. Hence, it is only during 
the change from the liquid into the solid state that the 
general rule of expansion and contraction is reversed. 

This property of water is of great importance. Lakes and rivers 
cool in winter by radiating and conducting heat to the colder 
air. As the surface water cools and contracts, it sinks, and is re- 
placed by warmer water from beneath, which in turn cools and 
sinks, until the whole depth of the water is reduced to 39 . Should 
this process continue until ice was formed, the ice, too, would sink, 
and accumulate at the bottom until the lakes and streams were 
converted into solid blocks of ice, which the heat of the succeed- 
ing summer could not melt. But, after reaching 39 , the water 
expands by cooling until after ice is formed. Hence, the ice-cold 
surface water and the ice are lighter than the deeper water, and 
form a floating blanket, which prevents to a great extent the escape 
of heat from the slightly warmer water beneath, and so preserves it 
in a liquid state through the winter- 
Evaporation is the process by which many liquids and 
some solids pass into a gaseous state at temperatures far 
below their boiling points. Evaporation is almost con- 



26 PHYSICAL GEOGRAPHY. 

stantly taking place at the surface of every sheet of water, 
snow, or ice, as well as at every moist surface in the world. 
It is made strikingly manifest when a damp cloth is hung 
in the air, for in a short time the cloth becomes dry — 
that is, the moisture evaporates and passes off into the air 
in the form of invisible water-gas, or vapor. 

Although evaporation takes place at temperatures much lower 
than the boiling point of water, the amount of energy rendered 
"latent" is about the same in both processes. Energy in the case 
of evaporation is supplied by the molecular motion of surrounding 
substances, which thus become cooler. This accounts for the sen- 
sation of cold when a rapidly evaporating liquid, as cologne or 
ammonia, is poured on the hand ; part of the energy employed in 
maintaining the temperature of the hand is drawn upon to re- 
arrange the molecules of the liquid and maintain it in a gaseous 
state ; this energy thus disappears as heat, or becomes latent. 

Mechanical Equivalent of Heat. — Exactly the same 
amount of energy is always required under similar condi- 
tions to increase the heat of a substance from one given 
temperature to another ; and conversely, in cooling from 
a given temperature to another, a body always liberates 
exactly the same amount of energy. The amount of 
energy required to raise the temperature of a pound of 
water i°, and to effect its corresponding expansion, is 
equal to that possessed by a one-pound weight striking 
the earth after a fall of 772 feet. This amount of energy 
is called the mechanical equivalent of heat. 

The enormous energy of heat is thus made manifest : a cubic foot 
of water (62^ pounds), can never be heated from the freezing to the 
boiling point except by the expenditure of enough energy to raise 
bodily a large locomotive engine and tender (43^ tons) 100 feet 
high into the air; and whenever a cubic foot of water simply cools 
from the boiling to the freezing point, enough energy is liberated 
to accomplish this same enormous lift. 

Refraction. — A ray of light passes through a trans- 
parent body in a straight line; but in passing obliquely 



SOME GENERAL LAWS OF NATURE. 



27 





from any transparent body to another of different density, 
as from air to water, water to air, or air to glass, the path 
of the ray is bent from a straight course. This bending is 
called refraction. 

Thus, suppose the ray ab (Fig. 5) to be passing obliquely from the 
air into the denser transparent substance, glass. 
At b part of the ray is reflected toward g ; 
part is absorbed by the glass, and the rest is 
refracted in the direction be. At c part of this 
is reflected back toward /z, and the rest, upon 
re-entering the air, is again refracted in the 
direction cd. If the surfaces of the glass are 
not parallel, but form the sides of a triangular prism, as in Fig. 6, 
then the incident ray ab, from the candle, will be refracted in the 

direction cd. As objects appear in the 
direction from which the ray enters the 
eye, the candle a would appear to an 
observer at d to be at x. If the inci- 
dent ray fall very obliquely on the re- 
fracting surface, it is totally reflected 
from that surface, and does not pene- 
trate it at all. Thus, when a ray from an object at the bottom of a 
pond makes an angle greater than 48 27' with a perpendicular to 
the surface of the water, it does not enter the air, but is totally re- 
flected from the surface toward the bottom again. 

Diffusion of Light. — If a ray of sunlight enters a 
completely darkened room through a small aperture and 
falls upon a screen, (1) the path of the ray becomes visi- 
ble from the illumination of the floating dust and air 
particles ; (2) a bright white image of the aperture is 
formed on the screen; and (3) the light, reflected from 
the dust and air particles, and from the image on the 
screen, is diffused throughout the room, and the outlines 
of objects in it become visible. 

Were it not for this general reflection and diffusion of light by 
the particles of the atmosphere, all shadows would be perfectly 
black, and all objects in shadow would be invisible. 



«& 



Fig. 6. 



28 



PHYSICAL GEOGRAPHY. 




The Spectrum. — If a triangular prism of glass be held 
in the ray between the aperture and the screen, the bright 

white image not only alters 
its position by the refraction 
of the ray, but it changes 
into an elongated, variously 
colored band (Fig. 7), the 
colors shading off impercept- 
ibly from red at one end, through orange, yellow, and 
green, to a pale blue or violet at the other end. This 
colored band of light is called the solar spectrum. 

The spectrum is explained by supposing that the sensation of 
color depends entirely upon the rapidity of the ether vibrations or 
waves, which produce light. When the rate of vibration is 392 
trillions to the second, the sensation of red is produced upon the 
eye. As the vibrations increase in rapidity, they give rise succes- 
sively to each of the color sensations of the spectrum. If the 
rapidity of vibration increases beyond that which produces the sen- 
sation of violet (757 trillions to the second), the eye is not affected, 
and they cease to be luminous. A ray of sunlight is composed of 
vibrations of all degrees of rapidity which collectively produce a 
white or colorless sensation. By refraction, the ether waves of 
different degrees of rapidity are separated, the more rapid waves be- 
ing bent, or refracted, more than the. slower ones; thus, the elon- 
gated band or image is produced, each part of which reflects to the 
eye waves of a different rapidity, and hence produces different color 
sensations. The color of any object in nature depends upon the 
rapidity of the ether waves which it is able to reflect or transmit to 
the eye ; thus, red glass absorbs the energy of all the luminous 
ether waves except that of the slowest, which it is able to transmit ; 
these rays produce the red sensation, and the glass appears of that 
color. A leaf, in the same way, absorbs all ether waves excepting 
those which on reflection excite the green sensation. This absorp- 
tion by different bodies of ether waves of certain length, and the 
transmission or reflection of those of other length, is called selective 
absorption. 

Magnetism and Electricity are peculiar states or con- 
ditions of matter, probably of the luminiferous ether, pro- 



SOME GENERAL LAWS OF NATURE. 29 

duced by the expenditure of kinetic energy upon it. The 
exact nature of these conditions is very imperfectly under- 
stood, but many of the peculiarities which they induce in 
ordinary matter have long been recognized. 

Under the influence of the magnetic condition, a 
body exercises an attractive or a repellent force upon other 
matter in its neighborhood, and is called a magnet. The 
neighborhood over which it exerts this force is called its 
magnetic field. 

A kind of iron ore (lodestone) is always magnetic, and attracts 
certain substances. Small pieces of iron, for instance, will move to 
the lodestone over short distances, and adhere to it, and while under 
its influence are themselves magnetic. Soft iron, however, loses 
this quality upon the removal of the lodestone, and is, therefore, 
called a temporary magnet. Hard iron and steel, on the contrary, 
retain the magnetic properties of the lodestone, and become perma- 
nent magnets. Magnetism is said to have been imparted to these 
bodies by induction. 

Poles of a Magnet. — The attraction of a magnet is 
not uniform throughout its length, but is greatest near its 
ends, which are called poles. Thus, if a magnet be laid 
among iron filings, they will adhere in great tufts to the 
ends or poles, but not to the center of the magnet. 
Whenever a body is magnetized, however small it may 
be, it exhibits this peculiarity of two poles, one at either 
end, with a region deficient in magnetism between them. 
One of the poles of magnets is called the positive (+), and 
the other the negative ( — ) pole, while the line joining the 
poles is called the axis of the magnet. 

Law of Polar Action. — If a permanent magnet be 
delicately balanced on a pivot at the center, so that it may 
swing freely, and either pole of a second magnet be suc- 
cessively presented to its two ends, it will be found that 
like poles (two -f- or two — poles) repel, while unlike poles 
(a ~j- and a — pole) attract each other. 



\o 



PHYSICAL GEOGRAPHY. 



Lines of Magnetic Force. — If one of the poles, say 
the -j- pole, of a strong magnet, be placed against the 
lower side of a horizontal plate of glass, on the upper 
side of which iron filings are scattered, the filings are 
magnetized by induction through the glass; and if the 
glass be lightly tapped, the filings tend to arrange them- 
selves in lines radiating from the portion of the glass im- 
mediately over the pole of the magnet. These are called 
lines of magnetic force. 




Fig. 8. 

This is caused by the attraction of the magnet beneath the glass 
for the opposite pole of each iron filing, which has been rendered a 
temporary magnet by induction ; hence, when the filings are jarred by 
the tapping on the glass, each points its greatest length or axis away 
from the locality of the bar magnet, thus producing the radiating 
lines. These are called lines of magnetic force, because, if a per- 
manent magnet, freely swinging on a pivot at its center, be set upon 
the glass, it will settle to rest with its axis parallel with the line or ray 
beneath it. and its — pole pointed toward the -f- pole of the magnet 
beneath the glass. The same thing would have. happened if the 
— pole of the bar magnet had been placed beneath the glass, ex- 
cepting that in that case the opposite (or -(-) pole of the swinging 
magnet would have pointed toward it. (Fig. 8.) 

The earth itself is a huge magnet ; one of its poles 
is at present located beneath the northern part of North 
America, and the other beneath the antarctic regions 



SOME GENERAL LAWS OF NATURE. 



31 




Fig. 9.— Magnetic Meridians in Northern Hemisphere. 

south of Australia ; but these poles very gradually change 
their position, though they seem to be confined to the 
arctic and antarctic regions respectively. The lines of 
magnetic force of this great terrestrial magnet are usually 
called magnetic meridians. The direction of the magnetic 
meridians in the northern hemisphere is represented by the 
heavy lines in Figure 9. Each of these magnetic merid- 
ians, like the lines of force in the iron filings, indicates the 
direction in which a freely swinging magnet will settle to 
rest at localities on that meridian. 

P. G.— 3. 



32 PHYSICAL GEOGRAPHY. 

Variation of the Compass. — The compass is essen- 
tially a freely swinging magnet. The pole of this magnet 
or " needle," which points to the magnetic pole in the arctic 
regions, is called its north or + pole. But the magnetic 
pole toward which the compass points, does not coincide 
with the end of the earth's axis or true north ; hence, the 
compass needle does not every-where indicate a true north- 
and-south line ; the angle which it makes with this line is 
called the variation of the compass. 

The chart shows that the variation is considerably west of true 
north over most of the Atlantic and its coasts, but to the east over 
the Pacific and its coasts. Possibly in consequence of a slow move- 
ment in the magnetic pole, the west variation in the United States is 
now increasing, while the east variation is diminishing. 

The cause of terrestrial magnetism is undetermined, 
but it is not improbable that it is induced by electricity 
generated in the luminiferous ether by the rotation of the 
earth. Magnetic storms, or unusual movements of the 
magnetic poles and meridians, producing sudden vibra- 
tions of the compass needle, and sometimes interfering 
with the working of telegraph lines, occur occasionally, 
and are most frequent at intervals of about eleven years — 
corresponding to times of great disturbance in the sun. 

The electrical condition of matter is rendered mani- 
fest in many ways. When in this condition matter is 
almost always heated, and sometimes so highly heated as 
to become luminous. Quite frequently the electrified body 
becomes a temporary magnet ; sometimes the body is 
thrown into sensible movement ; sometimes the movement 
is insensible (molecular), but sufficiently violent to over- 
come cohesion, and thus change the state of aggrega- 
tion of the body ; at other times, the movement may be 
violent enough to overcome chemical affinity, and thus 
cause the decomposition and entire disappearance of the 
electrified substance. Very frequently the collision of dis- 



SOME GENERAL LAWS OF NATURE. 33 

placed molecules gives rise to sound. The sound may 
vary in intensity, from the scarcely audible crack of a 
small electric spark, to the deafening crash of thunder. 

Development of Electricity. — The expenditure of 
energy in any manner and upon any substance seems to 
develop a greater or less amount of electricity ; even the 
bringing into simple contact of any two substances seems 
to result in a more or "less pronounced electrical condition 
in these substances. 

As simple contact of two substances develops an electrical con- 
dition, this condition is constantly being developed in nature, but 
its effects are not always seen because many substances transmit 
electrical energy readily to the earth, where it quietly diffuses itself. 

Conductors. — All substances transmit electrical energy, 
but in many substances its passage is almost instantaneous ; 
such substances are called good conductors or conductors. 
Among them are the metals, liquids, and hence all moist 
bodies, including living animals and plants. Non-conductors 
transmit electrical energy with extreme slowness ; such are 
glass, dry air, stone and earthy substances, and dry ani- 
mal and vegetable matter. 

An electrified conductor surrounded by non-conductors retains its 
electrical energy a long time, and is said to be insulated. If a piece 
of brass be electrified and held in the hand, its electrical energy 
passes through the body, imparting a more or less powerful shock, 
and escapes to the earth ; but if the electrified brass rest on a glass 
plate, and is surrounded by dry air, it is insulated. 

Positive and Negative Electricity. — Two kinds of 
electrical energy always make their appearance simultane- 
ously when a body is electrified, and are respectively 
called positive and negative electricity. Each kind pos- 
sesses a strong attraction for the opposite kind, but repels 
electricity of the same kind. It is convenient to conceive 
that both kinds of electricity exist in all bodies, but when 
present in equal quantities they neutralize each other, and 



34 PHYSICAL GEOGRAPHY. 

the body exhibits no electrical properties. By the appli- 
cation of energy, these electricities are conceived to be 
separated and kept apart, the body thus becoming electri- 
fied, positive electricity collecting at one extremity and 
negative at the other. 

When a body is electrified, it always electrifies surrounding 
objects by induction ; that is, the near neighborhood of an electrified 
body causes a separation of the two kinds of electricity in surround- 
ing objects, negative electricity collecting on the side of the objects 
which are nearest to the positive end of the electrified body, and 
vice versa. 

The Electric Spark. — Thus, there is a strong attrac- 
tion between the electricity at either end of the electrified 
body and the opposite kind of induced electricity on the 
nearest surfaces of surrounding bodies. These electrici- 
ties tend to unite, but the intervening air, being a non- 
conductor, resists their union. If the charge be suffi- 
ciently intense, however, the electricity forces a passage 
for itself, part of its energy being transformed, by the re- 
sistance of the air, into heat. This makes the. air par- 
ticles along its path white hot for the fractional part of a 
second, and produces a streak of white light through the 
air called an electric spark. 

The more dense and dry the intervening air, the greater must be 
the electric charge which is able to penetrate it, and the more in- 
tensely luminous and streak-like is the resulting spark. When the 
air is very thin or rare, the passage through it of an electric charge 
produces a more or less feeble glow rather than a bright spark, and 
the glow is often beautifully colored. An electric spark is generally 
accompanied by a crackling sound, more or less audible, as the 
spark is larger or smaller. The sound is simply the clash of the air 
particles upon each other as the air suddenly expands and contracts 
under the great but instantaneous change of temperature produced 
by the passage of the electricity. 



PART I. — THE EARTH AS A PLANET 



CHAPTER I. 

THE SOLAR SYSTEM. 



The heavens declare the glory of God; and the firmament shoiueth his handy- 
work. — Psalm xix : i. 

Fixed Stars and Planets. — By attentively observing 
the stars at night, it will be seen that they appear to 
move slowly across the sky. Observations upon several 
nights will convince one that most of the stars move to- 
gether, like bright spots in a solid, revolving sky. In 
consequence, the position of each star is fixed in relation 
to the others, and on this account they are called fixed 
stars. Occasionally stars and comets are seen whose ap- 
parent nightly movement across the sky is not in unison 
with that of the others; they shift their positions among 
the fixed stars from night to night, and are therefore 
called planets (wanderers). The sun is also a wanderer, 
and appears each morning during the year among a dif- 
ferent group of the fixed stars. 

The Solar System. — These movements led astronomers 
to suspect that the wanderers have a peculiar relationship 
to each other, and further investigation confirmed this sus- 
picion, as it is proved that the planets and comets revolve 
about the sun, thus forming a separate group of heavenly 
bodies, much nearer to us than any of the fixed stars. 

(35) 



^ 



PHYSICAL GEOGRAPHY. 



We call this separate group the solar system. It is prob- 
able that each of the fixed stars is really a sun, and the 
center of a separate system of planets, but so far from 
us that the planets are invisible. 

The Sun is the largest and most important body in the 
solar system. Its shape, like that of all the planets, is 
globular, or ball-like, but it differs from the planets in 
temperature, its surface being much hotter than the hot- 
test fire. The sun supplies the solar system with radiant 
energy, which becomes sensible as heat, light, and in 
other forms. 

The sun has a diameter of more than 866,000 miles, and weighs 
about 760 times as much as all the planets put together. The sun is 
largely composed of matter in a gaseous state, many substances ex- 
isting there in that form, with which we are familiar only as dense 
solids, such as the metals. It is thought that the heat of the sun is 
developed and maintained by the gradual contraction and conden- 
sation of its gaseous body, and to a lesser extent by its collision with 
very small solid planetary bodies called meteors. 



I THE SUN 1 




— THE PLANET MERCURY 
VENUS 
EARTH 
MARS 
JUPITER 
SATURN 
URANUS 
NEPTUNE 
Fig. io. — Relative Diameters of the Sun and Larger Planets. 

The Planets are bodies much smaller than the sun, 
around which, at different distances, they revolve, and 
from which they receive most of their light and heat. 
The path of a planet around the sun is called its orbit. 
The planets shine at night by reason of the sunlight which 
is reflected from their surface, while the sun and the fixed 
stars shine by their own light. 

There are more than two hundred and fifty planets, of which 
eight are vastly larger than the rest. The large ones are named in 



THE SOLAR SYSTEM. 



37 



the order of their distance from the sun : Mercury, Venus, Earth, 
Mars, Jupiter, Saturn, Uranus, and Neptune. Mercury, Venus, Mars, 
Jupiter, and Saturn are frequently visible from the earth as remark- 
ably large and brilliant stars. Uranus is so distant as to be barely 
visible to the naked eye, and Neptune can never be seen without a 
telescope. Most of the other planets are very small, and lie between 
the orbits of Mars and Jupiter. They are never visible to the naked 
eye, and are called planetoids or asteroids. 

Satellites. — Several of the planets are attended by one 
or more smaller bodies called satellites, or moons, which 
shine by reflected sunlight, and* re- 
volve around their respective plan- 
ets, as the planets revolve around 
the sun. 

Comets are planetary bodies 
which revolve about the sun in very 
elongated orbits. The mass of a 
comet is usually very small, but it 
is generally so widely distributed 
that its volume is often enormous. 
Comets shine chiefly by reflected 
sunlight, though some comets ap- 
proach the sun so closely that they 
become sufficiently heated to shine 
by their own light also. 

Comets are often composed of a com- 
paratively small, dense, or solid nucleus, 
surrounded by a less dense cloud or coma. There is frequently, 
though not always, an extension of the coma on one side of the 
comet, which forms the " tail." The tail is composed of matter in a 
state of extreme tenuity, and is often thousands of miles in length. 

Meteors. — - Millions of small fragments of matter, 
possibly the debris of disintegrated comets, revolve about 
the sun. Many of them enter our atmosphere, in which 
case the friction of the air on the rushing fragment de- 




Fig, ii. — Donati's Comet. 



38 PHYSICAL GEOGRAPHY. 

velops enough heat to ignite the fragment and render it 
luminous as a meteor, ox "falling star." 

Meteors are generally entirely consumed in the air, but sometimes 
a remnant of one reaches the earth's surface as a mass of stone and 
metal called an aerolite. These foreign bodies show considerable 
diversities of composition ; but in no case have they yet revealed 
the existence of any element not found on the earth. 

The Nebular Theory. — The sun and all its planets 
seem to be composed of the same kinds of matter. They 
are all globular in form. They all have a spinning motion 
in the same direction. The planets all move around the 
sun, and most of the satellites around their respective 
planets in the same direction. The sun itself appears to 
be a very hot, gaseous body, which is gradually cooling 
and contracting in volume. These considerations have led 
to the nebular theory of the formation of the solar system. 
According to this theory, all the matter composing the 
various members of the solar system was once so hot that 
it existed as a single enormous cloud or nebula filling all 
the space within the orbit of the most distant planet. As 
the nebula cooled and contracted, it acquired a rotary or 
spinning motion, and threw off successive rings, each of 
which, cooling and contracting about its densest point, as- 
sumed at length the form of a spinning, globular planet, 
revolving about the parent mass of nebula, which we call 
the sun. Several of the planets, in cooling, are supposed 
to have thrown off secondary rings, which condensed into 
satellites, or moons. 

The Earth, upon which we live, is one of the eight 
larger planets, — the fifth in point of size, and the third in 
point of distance from the sun. The earth is attended by 
a secondary planet, called the moon, which revolves about 
the earth as the earth revolves about the sun. 

Shape of the Earth. — The earth is nearly round or 
globular in shape. If it were exactly round, its shape 



THE SOLAR SYSTEM. 



39 




Fig. 12. 



would be that of a sphere, but as it is slightly flattened on 
two opposite sides, its shape is that of a spheroid. If a 
person stands in the midst of a vast plain, or on the deck 
of a vessel at sea, the surface of the earth appears to be 
flat, and stretches away in every direction to a line where 
it seems to meet the sky ; this line is called the horizon. 
The circular area embraced by the horizon enlarges as 
the observer ascends, but 
even from the greatest height 
ever attained by balloonist 
(Fig. 12), the visible portion 
of the earth forms such an 
exceedingly small propor- 
tion of its whole surface 
that the curvature is entirely imperceptible. 

But the earth can not be flat, for mariners, by sailing continuously 
in the same general direction, have at last found themselves at their 

starting-point. That the surface 
is curved, is proved by the man- 
ner of disappearance of a ship 
upon reaching our horizon ; first 
the hull, or body, of the vessel 
sinks out of view, then the lower 
sails disappear, and finally the highest parts of the masts sink be- 
neath the horizon, That the curved surface is nearly that of a 
sphere is proved by the circular shadow which the earth invariably 
casts on the face of the moon at the time of a lunar eclipse. Now, 
a sphere is the only body which can cast no other than a circular 
shadow. Finally, very careful measurements upon the earth, and 
observations upon the fixed stars, have proved conclusively that the 
shape of the earth is spheroidal — but very nearly spherical — 299 of 
the shortest diameters being equal to 298 of the longest diameters. 

Size of the Earth. — The length of the shortest diam- 
eter of the earth is about 7,900 miles. The greatest 
diameter is about 26 miles longer. From these diameters, 
it follows that the greatest distance around the earth, or 











(4°) 




THE SOLAR SYSTEM. 4 1 

its circumference, is about 25,000 miles, while the total 
area of the earth's surface is 197 millions of square miles. 

These figures are much too large to convey a definite impression. 
The vastness of the earth may better be appreciated by considering 
that it would take a railway train moving a 
mile a minute, 17 days and nights of continuous 
travel to complete the greatest distance around 
it ; and that there is room on its surface for 
fifty-five countries as large as the United 
States. Large as the earth seems to us, it is 
greatly exceeded in size by four of the other Fig. 14.— Relative 
planets, while the surface of the sun has more Areas, 

than ten thousand times its area. 

Internal Temperature of the Earth. — The occur- 
rence in many localities of springs of hot, often boiling, 
water, and of volcanoes discharging steam and molten 
rock or lava, leads to the belief that the interior of the 
earth is very hot. Observations in mines, wells, and deep 
borings invariably indicate an increase of temperature with 
an increase of depth. The rate of this increase varies 
greatly in different localities, but the average is about i° 
for every 50 feet. At this rate of increase, a temperature 
sufficient to melt any known substance would be attained 
at a depth of 30 or 40 miles. 

The Density of the earth tends to confirm the belief 
in a very high internal temperature. Calculations prove 
that the earth weighs 5 ]/ 2 times as much as a similar 
globe of water. The surface rocks weigh from 2]/ 2 to 3 
times as much as water. The pressure to which such 
rocks would be subjected at great depths would so greatly 
increase their density that we should expect the specific 
gravity of the whole earth to be much greater than 5^. 
There must be some expansive force within, which parti- 
ally counteracts the pressure. Heat is the only force we 
know of that will do this. 




42 PHYSICAL GEOGRAPHY. 

Condition of the Interior. — The indications of a 
great internal heat, together with many facts in geology, 
lead many to believe that the earth is essentially a great 
globe of molten matter, on whose surface a cool, solid 
scum, or crust, has formed, which is comparatively thin — 
perhaps a hundred miles or more. 
Other phenomena have been held to 
indicate that the globe throughout is 
as rigid as steel. Those who hold this 
opinion believe it to be possible that 
the great weight of the overlying rocks 
may prevent expansion, which accom- 
Fig. 15.-A crust 100 panies the liquefaction of all known 

Miles Thick. rQckS) and thus retain the interior f 

the earth in a solid form, at a temperature far above its 
melting point. Whether the great interior of the earth is 
or is not liquid by reason of its heat, it seems certain that 
the rocks at no great depth are capable of flowing as if 
they were plastic — like thick tar. 

This peculiarity would result from inequalities in pressure on adja- 
cent regions. The weight of a few miles' thickness of the earth's 
crust is great enough to crush any known rock to powder, but a 
block of deeply buried rock can not fall to pieces as powder because 
of the side pressure of adjacent rocks. If, however, the pressure on 
any side should become less than that on the block, the latter would 
be more or less flattened, a portion of its substance being forced into 
the region of diminished pressure. Hence, no hollow places, caves, 
open cracks, or fissures can exist at great depths in the earth, for 
the enormous pressure would cause the adjacent rocks to "creep" 
or flow into the cavity and fill it. 



CHAPTER II. 

MOVEMENTS OF THE EARTH. 

And God said, Let there be lights in the firmament of the heaven to divide the 
day from the night; and let them be for signs and for seasons, and for days and 
years. — Genesis i : 14. 

Movements of the Earth. — The earth, which seems 
to us so solid and immovable, is really in constant and 
very rapid motion. It has a spinning motion, called rota- 
tion, on one of its diameters ; and a much faster motion, 
called revolution, in its orbit around the sun. 

We can not perceive these motions by observing objects in our 
neighborhood, because the whole earth moves along smoothly and 
noiselessly, carrying the atmosphere and all objects on its surface 
along with it. They thus preserve their relative positions as they 
would if the earth were at rest. It was only by carefully observing 
the sun and the fixed stars that it was discovered that their move- 
ments in relation to the earth are only apparent, and are caused 
by the movements of the earth itself. 

Rotation. — The earth spins, or rotates, at a nearly 
uniform rate of speed, upon its shortest 
diameter, called its axis. The ends of 
the axis are called the poles. A line 
around the earth midway between the 
poles is called the equator. One of the 
results of rotation is the succession of 
day and night. The sun shines upon 
but one half of the earth at a time. The 
other half, being turned away from the sun, is in dark- 
ness. As the earth rotates, each point on its surface is 
carried successively into the light and into the darkness, 

(43) 




44 PHYSICAL GEOGRAPHY. 

one day and one night, or about twenty-four hours, mark- 
ing one complete rotation. 

As the circumference of the earth is about 25,000 miles, and as 
the earth completes one rotation in twenty-four hours, it follows that 
a point on the equator moves at a speed of over 1,000 miles an hour. 
The speed of rotation is of course less at points on the surface 
nearer to the poles, and at the poles themselves is very slight, just 
as in a rotating wheel a point on the tire moves faster than a point 
on the hub. 

Spheroidal Form of the Earth caused by Rota- 
tion. — The inertia of all rotating bodies gives them a ten- 
dency to fly away from the center. This tendency is 
called centrifugal force, and it increases with the speed of 
rotation ; hence it is greater at the equator than toward 
the poles. In- obedience to this force, the equatorial 
regions bulge out and the polar regions draw slightly 
nearer together, producing the spheroidal form of the 
earth. 

Direction. — The earth always rotates in the same gen- 
eral direction, thereby affording the standard by which all 
terrestrial directions are determined. The direction in which 
the earth rotates is called east; it is nearly that in which 
the sun first appears every morning. The rotation of the 
earth to the east causes the sun and other heavenly bodies 
to appear to move across the sky in the opposite direc- 
tion; this direction is west. If we stand with extended 
arms, facing east, our arms will be parallel with the earth's 
axis. The direction in which our left arm points is north, 
and the end of the axis in that direction- is called the 
north pole. The direction in which our right arm points 
is south, and the end of the axis in that direction is called 
the south pole. 

But the sun seldom rises exactly in the east, and it is therefore 
customary to reckon direction from the north, which can be accu- 
rately determined by observations on the North Star, or Polaris, a 



MOVEMENTS OF THE EARTH. 



45 






M* 



. r 



fixed star situated almost directly over the north pole. For this 
reason it is often called the Pole Star. This star is of course only 
visible at places north of the 
equator, and can be found 
any clear night by reference 
to two stars, called ' ' the point- 
ers," in the constellation of 
"The Dipper." The north 
and south directions are in 
most places only approxi- 
mately indicated by the com- 
pass (page 32). 

Location. — The earth 
always rotates upon the Flg ' I7 ' 

same axis; hence, the ends or poles of this axis mark 
two fixed points. Upon these two points depends a sys- 
tem of meridians and parallels, by means of which the 
location of any point on the surface of the earth may be 
exactly described. 

Meridians are lines conceived to be drawn on the 
earth's surface directly from one pole to the other. Their 
direction is exactly north and south. Two meridians, ex- 
actly opposite each other, unite to form a great circle, 
which divides the earth's surface into an eastern and a 
western half, or hemisphere. 

The Equator. — The position of the equator depends 
upon the positions of the poles, since it lies just half-way 

between them. Its direction is 
exactly east and west. The 
equator is also a great circle, 
and divides the earth's surface 
into a northern and a southern 
hemisphere. 

Parallels are lines conceived 
to be drawn around the earth in the same direction as (par- 
allel with) the equator. They thus extend east and west. 




Fig. 18. — Meridians and 
Parallels. 



46 PHYSICAL GEOGRAPHY. 

Parallels divide the earth's surface into unequal parts, and 
are called small circles. 

Longitude is the angle which a meridian makes with 
some other meridian, assumed as the initial, or standard, 
meridian. Any meridian may be assumed as the standard, 
but the one passing through the observatory at Green- 
wich, England, is usually adopted. Longitude is reckoned 
in degrees and parts thereof, east and west from the stand- 
ard meridian through 180 or half-way round the earth. 
Hence, the meridian of 180 east longitude coincides with 
the meridian of 180 west longitude. Longitude thus fixes 
the position of the meridian of any place on the earth's 
surface with respect to the standard meridian. 

Since the meridians approach each other as they near the poles, 
the length of a degree of longitude decreases from the equator, 
where it is ^|(,th as long as the equator, or 69^ miles, to the poles, 
where it is nothing. The longitude of a place is reckoned by ob- 
serving the difference of time between that place and the standard 
meridian. The earth makes a complete rotation, that is, turns east- 
ward through 360 , in 24 hours; hence, it turns eastward 15 in one 
hour, or i° in four minutes. Therefore, at noon on a certain 
meridian it is four minutes before noon on a meridian i° to the west, 
and four minutes after noon on a meridian i° to the east. Noon 
may be approximately determined by observing when the sun 
is half-way between the eastern and western horizon. By com- 
paring a watch keeping the time of the standard meridian with the 
noon at any place, thus determined, the dif- 

eference of time is obtained. If the time by 
the watch is after noon, the longitude of the 
place is west, if before noon its longitude is 
T east, of the standard meridian, and as many 
i • 1 -i-rr r 

degrees as 4 is contained in the difference 01 
time in minutes. 
Latitude is the angle included be- 
Flg ' ig ' tween the radius of curvature of the 

earth's surface at any parallel and the plane of the equator. 
The angle LAT is the latitude of parallel LX. Latitude 



MOVEMENTS OF THE EARTH. 



47 



NO. 


Mir 






9tp 


aof 










~~z°° 









7~^60° 
















/° / \ 50 


n 














/•?/t, \40° 


"5: 








Plahe, 


^ 


Equator I 











Fig. 20. 



is expressed in degrees and parts thereof, and is reckoned 
north and south from the equator to the poles, 90 north 
or south latitude corresponding to the north or the south 
pole respectively. Latitude thus fixes 
the position of any parallel with re- 
spect to the equator. 

Degrees of latitude are 68^ miles long- 
near the equator, but they increase grad- 
ually toward the poles to a length of 69^ 
miles. The reason for this slight increase 
is the spheroidal form of the earth. The 
convexity of its surface decreases from the 
equator toward the poles ; therefore, the 
radius of curvature increases in the same 
direction. Two radii with the same degree 
of divergence are thus farther apart in polar than in equatorial 
regions, as indicated in Fig. 20, in which the spheroidal form of 
the earth is greatly exaggerated. Latitude may be reckoned by 
observing the angular distance between the horizon and the celestial 
pole. The celestial poles are points in the heavens directly over the 
poles of the earth or terrestrial poles, the approximate position of 
the north celestial pole being marked by the pole star Polaris. At 
the equator, where the latitude is zero, both the celestial poles lie on 
the horizon. As one travels from the equator toward one of these 
poles, the horizon sinks away from it, or the celestial pole seems to 
rise above the horizon. Half-way from the equator to the terrestrial 
pole, or in latitude 45 , the celestial pole is half-way to the zenith or 
45 above the horizon. At the terrestrial pole, in 90 latitude, the 
celestial pole is overhead, or 90 from the horizon. Hence, the 
angular distance of the celestial pole above the horizon is the same 
as the observer's latitude or angular distance from the equator. 

Practical Use of Longitude and Latitude. — Since 
longitude fixes the position of any meridian with respect 
to the standard meridian, and latitude the position of any 
parallel with respect to the equator, it follows, if the longi- 
tude and latitude of any place are known, the position of 
the place on the earth's surface is definitely fixed. Thus, 

if a place is in jy° 03' W. long. Gr. and 38 53' N. lat, it 
p. g.— 4 . 



48 PHYSICAL GEOGRAPHY. 

is known to be at the intersection of the meridian 77 03' 
west of Greenwich, with the parallel 38 53' north of the 
equator. 

Revolution. — In addition to rotation, the earth has a 
forward movement through space in its orbit. The 
shape of the orbit is due to the action of two great forces. 
These are: (1) the gravitation of the sun, which tends 
to deflect the path toward that luminary; and (2) centrif- 
ugal force, which resists any deflection of the path from 
a straight line. As a result of these two forces, the orbit 
of the earth becomes nearly circular around the sun. 
c „ The movement of the earth in its orbit is 

Earth 

,jS* ° called its revolution to distinguish it from its 

"\ movement of rotation. 

While nearly circular, the exact shape of the 
orbit is that of an ellipse, the sun being situated, 
not at the center, but at one of the foci. Owing to 
this fact, the distance from the earth to the sun varies about three 
million miles in different parts of the orbit. The mean distance is 
about 91^ million miles, — a distance which a railroad train, moving 
a mile a minute, would require 175 years to traverse. 

Orbital Velocity. — It takes about 365*^ solar days 
for the earth to complete one revolution around the 
sun. This interval of time constitutes a year. In order to 
traverse its orbit in 36$ *%. days, the earth moves at the 
enormous velocity of nearly 1,100 miles a minute, but its 
speed is not uniform ; it moves faster when nearest the 
sun (perihelion), and slower when 
most distant (aphelion). *.;^*a 

At each point of the orbit, A BCD, the \ / 

inertia of the earth urges it in the direction bo # ? D 

of the tangents, /, (centrifugal force,) while J\ j \ 

gravity draws it toward the sun, s. At two * ""■- I Jxr*^ 1 

points in the orbit, B, D, gravity acts at F *. 22 

right angles to the centrifugal force, and 

neither adds to nor diminishes the speed, but only bends the line of 



Sun 

■■■ Orbtt. 

Fig. 21. 



MOVEMENTS OF THE EARTH. 49 

motion into a curve. At all other points gravity either increases or 
retards the speed ; thus, at A it helps the earth forward. This in- 
creased speed adds to the centrifugal force, and not only enables the 
earth to resist being drawn into the sun, but, after passing B, to in- 
crease its distance from the sun. At C, however, gravity retards the 
speed and diminishes the centrifugal force, so that in the end gravity 
prevails and retains the earth in its orbit. The varying velocities of 
the earth are so nicely adjusted to its distance from the sun that the 
amount of heat it receives in passing through equal angular dis- 
tances of its orbit are exactly equal, the greater velocity in perihelion 
just compensating the greater distance in aphelion. Thus, the 180 
of the orbit, xBy, are nearer the sun, and hence hotter than the 180 
xDy, but the amount of heat received by the earth is equal in each 
segment, since less time is occupied in passing over the former than 
the latter. 



P lane of Ec lip tic j£> th e : 

—>SUN 



Fig. 23. 

Inclination of the Earth r s Axis. — The absolute di- 
rection of the earth's axis, at all points of the orbit, is 
nearly the same. This direction makes an angle of about 
66y 2 ° with the plane of the ecliptic, — a plane passing 
through the earth's orbit and the sun's center. The axis, 
therefore, leans about 23^° (90 — 66*4°) out of a perpen- 
dicular to the plane of the ecliptic. This angle constitutes 
the inclination of the axis. 

Zones. — The general distribution of the sun's heat over 
the earth's surface has occasioned its division into five belts, 
or zones, — a torrid, or hot zone, embracing the equatorial 
region ; two frigid, or cold zones, including the region about 
either pole ; and a temperate zone between the torrid zone 
and either frigid zone. 

Any surface upon which the sun's rays fall perpendicularly is 
hotter than it would be if the rays fell obliquely, because more rays 



50 



PHYSICAL GEOGRAPHY. 




fall upon it. Thus, ab, cd, and ef are equal spaces, and let r repre- 
sent the parallel rays of the sun falling on them. It is seen that cd, 
upon which the rays fall nearly perpendicu- 
larly, receives almost three times as many 
rays as either ab or ef where they fall very 
obliquely. This fact explains why morning 
and evening are cooler than noon, a and/" in- 
dicating the position of places with reference 
Fi s- 2 4- to the sun's rays in the morning and evening, 

and cd the position during the middle of the day ; ab also indicates 
the position of regions near the poles of the earth, where the sun's 
rays always fall obliquely. Hence, these regions are much colder 
than those near the equator, where the rays fall obliquely only in 
the morning and evening. 

The Tropics and the Polar Circles. — The lines 
bounding the zones are parallels of latitude : those bound- 
ing the torrid zone are called tropics, and those bounding 
the frigid zones are called polar circles. The position of 
these depends upon the inclination of the earth's axis. 

In the diagram, A BCD represents the earth at four points in its 
orbit around the sun, S. At A the sun at noon is in the zenith of 
the equator y // . Each day, 
as the earth advances in its 
orbit, the inclination of the 
axis causes places farther 
and farther from the equator 
to be presented to the verti- 
cal rays of the noon sun, 
until, at B, the sun at noon 
is vertical over z, which is 
as far from the equator, y, 
as the north pole, N, is dis- 
tant from x ; that is, 23^°, or the angle of inclination of the earth's 
axis. As the earth passes onward in its orbit, the axis assumes daily 
a position in relation to the sun more nearly similar to that at A, and 
the earth presents to the vertical rays of the noon sun points nearer 
and nearer to the equator, until, at C, the sun is exactly over the 
equator again. Through the other half of the orbit the. same phe- 
nomena take place, but on the other side of the equator. Thus, 




Fig. 25. 



MOVEMENTS OF THE EARTH. 5 1 

the only part of the earth which ever receives the vertical rays of 
the sun lies between the parallels of 23^° latitude on either side of 
the equator ; hence, these parallels are taken as the limits of the 
hottest or "torrid" zone. The parallel on the north is called the 
tropic of Cancer, and the one to the south the tropic of Capricorn. 
They are called tropics (turnings), because over them the sun ap- 
pears to turn and retrace his course toward the equator. It will be 
observed in the diagram that when the earth is at B or D, the 
region within 23^° of one of the poles is in darkness, and hence 
receives no light or heat during an entire rotation of the earth. 
These regions which, during at least one day of the year receive 
no heat rays, and during the rest of the year receive them only 
very obliquely, must be the coldest parts of the earth ; hence, the 
parallels 23^° from the poles, or in latitude 66)4°, are taken as the 
limits of the frigid zones. The polar circle near the north pole is 
the Arctic Circle ; that nearest the south pole is the Antarctic Circle. 

Length of Days and Nights. — Owing to the inclina- 
tion of the earth's axis, there are but two points in the 
orbit where the light of the sun reaches both poles of the 
earth at the same time. These two points, (A, C, Fig. 
25,) are called the equinoxes (equal nights), for when the 
earth occupies either of these positions in its orbit, the 
days and nights are every-where of equal length. At all 
other points in the orbit, as B and D, the light of the 
sun extends beyond one pole, but fails to reach the other. 
As a result, more than half of one polar hemisphere is 
illuminated, and its days are longer than its nights, while 
less than half of the other hemisphere is illuminated, and 
consequently in that hemisphere the days are shorter than 
the nights. 

By referring to the last diagram, and remembering that the earth 
is constantly rotating upon its axis as well as moving around the sun 
in its orbit, it will be seen that the days and nights are always of 
equal length (12 hours) at the -equator, but at all other places they 
are of unequal length, excepting when the earth is at the equinoxes. 
The days are shortest in the northern hemisphere and longest in 
the southern when the earth is at B. The reverse is the case six 
months later when the earth is at D. At these positions, called sol- 



52 PHYSICAL GEOGRAPHY. 

stices, it is continuous day at one polar circle, and continuous night 
at the other, during a complete rotation of the earth, or 24 hours, 
while at the poles it is either continuous day or continuous night 
while the earth is passing from A to C, or for six months. 

The Seasons. — The succession of the seasons depends 
upon the revolution of the earth, together with the incli- 
nation of its axis. The earth is in the position C (Fig. 
25) on the 2 1 st of March. This is the vernal equinox, and 
the days and nights are every-where of equal length. As 
the earth moves forward in its orbit, the north pole begins 
to incline toward the sun, the days lengthen in the 
northern hemisphere and shorten in the southern ; and, 
since the sun reaches the zenith of points north of the 
equator, the heat increases in the northern hemisphere 
while the cold increases in the southern. After 92^ days, 
or about June 21st, the sun reaches the zenith of the 
tropic of Cancer (D). This is the summer solstice in 
the northern hemisphere, which receives the sun's rays 
most perpendicularly, and the winter solstice in the southern 
hemisphere, upon which the sun's rays fall most obliquely. 
As the earth advances, the sun day by day reaches the 
zenith of points nearer the equator, and the days grow 
shorter in the northern hemisphere and longer in the 
southern. After 92^ days the earth reaches the autum- 
nal equinox (A) about September 22. This is the be- 
ginning of spring in the southern hemisphere. Moving 
onward, the earth gradually presents its southern pole to 
the sun, while the north pole enters its annual period of 
cold and darkness. For 90 days the days in the northern 
hemisphere grow shorter until the winter solstice is reached 
about December 21 (B). This is the summer solstice, 
however, in the southern hemisphere, the time of its 
longest day and most direct exposure to the sun, which 
reaches the zenith of the tropic of Capricorn. Passing on 



MOVEMENTS OF THE EARTH. 



53 



from this point in its orbit, the earth gradually withdraws 
its south pole from the sun until, after 90 days, it reaches 
the position of the vernal equinox again. 

Projections. — It is impossible to represent with perfect 
accuracy the curved surface of the earth upon the flat sur- 
face of a map. Many arrangements, called projections, of 
the meridians and parallels, have been invented, each of 
which reduces to a minimum some one or more of the 
inevitable inaccuracies, while it may exaggerate others. 
When any feature of the earth's surface, therefore, is to 
be illustrated by means of a map, it is best to select from 
the many map projections one that is designed to show 
that special feature most accurately. Various maps in 
this book are thus drawn in three different projections : 
(1) Mercators, (2) Lamberts, and (3) Polar. 

In Mercator's Projection (Fig. 26). the meridians and parallels 
are straight lines crossing at right angles, and the spaces be- 
tween them are so proportioned that any continuous direction on 
the earth's surface may be 
represented by a straight 
line on the map. Hence, 
every change in direction 
of any line on the map 
represents a corresponding 
change in direction of the 
line it represents on the 
earth's surface. Therefore, 
this projection is useful when 
relative directions in differ- 
ent parts of the earth are to 
be compared, as the direc- 
tions of different ocean currents, etc. The scale of the map, how- 
ever, is not uniform, but increases rapidly and irregularly from the 
equator toward the polar regions. Thus, Greenland is represented 
as larger than South America ; whereas, it is really less than one 
eighth as large. 

In Lambert's Projection (Fig. 27), equal areas on the map repre- 
sent equal areas on the earth's surface. It may, therefore, be used 




Fig. 26. 



54 



PHYSICAL GEOGRAPHY. 




when areas are to be compared. The equator and the central me- 
ridian (not drawn in Fig. 27) of each hemisphere are straight lines 

drawn at right angles, but 



the parallels and all other 
meridians are dissimilar 
curved lines, the bounding 
meridians of each hemi- 
sphere joining at the poles 
to form circles. The space 
between meridians de- 
creases toward the bound- 
ary of each hemisphere, but 
that between parallels increases in such proportion that all the sub- 
divisions between any two parallels have the same area. 

In Polar Projection (Fig. 28), the observer is supposed to be imme- 
diately over one of the poles of the earth, in the center of the map, 
and to be able to see at a glance the whole polar hemisphere from 
the pole to the equator (represented by the heavy circular line in the 
diagram). The parallels and the equator are indicated by concentric 
circles, and the meridians by straight lines radiating from the pole 
to the equator. This projection is specially adapted to show the true 
relative position of features on 



Fig. 27. 



opposite sides of the same 
polar hemisphere. The two 
polar hemispheres may be sep- 
arately shown in two circles 
having the north and the south 
poles respectively for their cen- 
ters, and each terminating at 
the equator. The Isothermal 
and Isobaric charts (pages 63 
and 85) are so drawn because 
these features form a complete 
system in each polar hemi- 
sphere, and may therefore be 
represented separately. But 



4 







Fig. 28. 



when features to be compared are not entirely embraced in one 
polar hemisphere, the whole surface of the earth may be shown in 
this projection, by dividing the concealed hemisphere into any 
number of equal sectors, extending from the equator to the pole, and 
arranging these as points around the equator. 



PART II. THE ATMOSPHERE. 



CHAPTER III. 

COMPOSITION, WEIGHT, AND HEAT. 

The Lord hath his way in the whirlwind and in the storm, and the clouds are 
the dust of his feet. — Nahum i: 3. 

The Atmosphere is the outer covering of the earth. 
It is composed of a light and gaseous substance called air, 
and completely envelops the solid and liquid parts of the 
planet, filling the deepest depressions, and extending above 
the tops of the highest mountains. 

Composition of Air. — Air is simply a mixture of 
several elements and compounds. The most important of 
these are the four invisible gases, nitrogen, oxygen, water 
vapor, and carbonic acid. 

The Nitrogen and Oxygen form the great bulk of the 
atmosphere (about |-f ths of the whole) in the proportion of 
four measures of nitrogen to one measure of oxygen. The 
oxygen is the element useful to life. The nitrogen simply 
dilutes the oxygen. 

Water Vapor is always present in the atmosphere. In 
extremely cold air its quantity is very minute, but in hot 
air it may form almost ^g-th P ar ^ °f the whole. Vapor is 
supplied to the atmosphere by evaporation from moist 
surfaces, and is the source of clouds, rain, snow, hail, and 
dew, 

(55) 



56 PHYSICAL GEOGRAPHY. 

Carbonic Acid is a compound of carbon and oxygen. 
It is given off to the atmosphere in the breath of animals, 
by the decay of animal and vegetable matter, and by the 
combustion of fuel. In amount it varies from 3 to 20 
measures in 10,000 measures, but when present in the 
latter quantity the air is unfit to breathe, producing stupe- 
faction, and eventually death. Plants are largely com- 
posed of carbon, which they obtain from the carbonic acid 
of the air, and thus prevent its undue accumulation in the 
atmosphere. 

In addition to the above, there are generally present in the 
atmosphere traces of ammonia and of many other gases, besides 
multitudes of minute solid dust particles and microscopic living 
germs. The dust motes are visible when a ray of sunlight crosses 
a darkened room (page 27). 

Weight of Air. — Although invisible and comparatively 
}ight, air is a veritable substance and has appreciable 
weight. This is made evident by the resistance it offers to 
the movements of an open fan, and, when it is itself in 
motion, by its effects upon the sails of a ship or a windmill. 
If a hollow glass globe holding a cubic foot is emptied 
of air and carefully weighed at sea-level, when the air is 
again let in it will be found to, weigh about 1 ^ ounces 
more than before. This increase is manifestly the weight of a 
cubic foot of air. An equal bulk of water weighs about 
840 times as much. It is gravitation, or the weight of air, 
which holds the atmosphere to the earth, just as it is the 
weight of water which holds the oceans in their beds. 

Pressure, — The weight, not of a cubic foot of air, but 
of the whole atmosphere resting upon any specified area, 
creates thereon a pressure called the atmospheric pressure. 

The Barometer is an instrument used to ascertain the amount of 
atmospheric pressure. The barometer in most common use consists 
of a glass tube about three feet long, closed at the upper end and 
open below. The air being entirely taken from the tube, the 



PRESSURE OF THE ATMOSPHERE. 



57 



open end is immersed in a little basin of mercury (Fig. 29/;). It is 
evident that the atmospheric pressure upon the exposed surface in 
the basin will force a column of mercury up . 

into the vacuous tube, until the weight of the 
column becomes just equal to that pressure. <■ 

When suitably mounted, as in Fig. 29^, there 
is attached to the tube a graduated scale for 
ascertaining the length of the mercurial 
column. The length of a column required to 
balance atmospheric pressure at sea-level is 
found to vary constantly, but on the average 
it is about 30 inches long ; and since a column 
of mercury one inch square and 30 inches 
long weighs 14^ pounds, it follows that this 
is the average weight, or pressure, of the at- 
mosphere on each square inch of the earth's 
surface, at sea-level. 

Density. — Being gaseous, air is elas- 
tic ; even a slight pressure squeezes it 
into less space and renders it denser, 
but upon the removal of pressure, it im- 
mediately expands and becomes less 
dense. As every portion of the atmos- 
phere sustains the pressure of the por- 
tion over it, the lower part is more 
heavily compressed than the upper. For 
this reason the atmosphere is densest 
near sea-level, and becomes less dense 
as the distance above that level in- 
creases. » Fig. 29. 

Height. — The height or depth of the atmosphere has 
never been determined. Observations with the barometer 
indicate that with each ascent of about 3 y 2 miles, one half 
of the former weight of the atmosphere is left below. 
The density decreases, of course, in the same ratio. At 
a height of 7 miles, the atmosphere is scarcely dense 
enough to sustain human life. At 50 miles it is no longer 



58 PHYSICAL GEOGRAPHY. 

dense enough to reflect the rays of the setting sun to cause 
twilight. But meteors have been observed at a height of 
200 miles, and as they are caused by the rush of solid 
bodies through the air, the atmosphere, though greatly 
rarified, must exist at that elevation. 

Since atmospheric pressure decreases with an increase of eleva- 
tion, the mercury in a barometer falls as the instrument is carried 
upward. To moderate elevations, the rate of this fall is about T Vh 
of an inch for each 100 feet of ascent. The barometer may there- 
fore be used to determine the relative heights of two or more places. 

Uses of the Atmosphere. — Besides supplying oxygen 
to animal, and carbon to vegetable life, the atmosphere 
contributes to the habitability of the globe in three im- 
portant particulars: (1) It accumulates the heat of the 
sun near the surface of the planet. (2) The condensation 
of its vapor is the only natural source of the world's 
supply of fresh water. (3) Its movements, or the winds, 
tend to equalize temperatures over the surface of the 
earth, and by its movements moisture is brought from the 
sea and distributed over the land. 

Heat of the Atmosphere. — The sun may be regarded 
as our sole source of heat. The stars, the heated interior 
of the earth, the friction of meteors and of terrestrial 
bodies, chemical combinations, etc., are all sources of 
heat, but the combined amount so produced is trifling in 
comparison with that received from the sun, and may be 
disregarded. 

How Imparted. — The heat of the sun is imparted to 
the atmosphere in four ways: 

(1) Directly. — In their passage through the atmosphere 
the sun's rays lose about one third of their heat. This is 
absorbed by the atmosphere and raises its temperature. 

(2) Contact. — About two thirds of the energy of the 
sun's rays are not absorbed in its passage through the atmos- 



HEAT OF THE ATMOSPHERE. 59 

phere. This energy reaches the earth's surface and raises 
its temperature, and evaporates water. Contact with this 
warmed surface warms the lower atmosphere. 

(3) The liberation of latent heat upon the condensation 
of vapor makes the surrounding air somewhat warmer 
than it otherwise would be. 

(4) Radiation from the earth's surface. — The earth's sur- 
face, being warmed by the sun's rays, immediately radiates 
heat back toward space, but these slowly vibrating rays 
from the earth have not the penetrative power of the rays 
of the sun, and are largely absorbed by the lower atmos- 
phere, which is thereby warmed (page 21). 

The atmosphere thus causes heat to accumulate near the earth's 
surface ; it allows a great portion of the sun's rays to enter, but re- 
tards the escape of heat. Without this property of the atmosphere 
no life of any kind could exist on the earth's surface, since its tem- 
perature, even under the direct rays of a tropical sun, would proba- 
bly never rise above zero. 

Distribution of Temperature. — Since the atmosphere 
is warmed by contact with and radiation from the earth's 
surface, the lowest portion of the atmosphere is the warm- 
est; and the warmest part of the lower atmosphere is the 
part over the warmest portions of the earth, or the por- 
tions in the neighborhood of the equator. There is, there- 
fore, a vertical and a horizontal variation of atmospheric 
temperature. 

Vertical Variation. — Many observations of tempera- 
ture made at various elevations in different parts of the 
earth, indicate that the atmosphere grows colder at the 
average rate of i° Fahrenheit for each 300 feet of increased 
elevation. 

Horizontal Variation. — Since the sun is vertical over 
the northern hemisphere in our summer, and over the 
southern hemisphere in our winter, the amount of heat 
received by either hemisphere during the two seasons is 



60 PHYSICAL GEOGRAPHY 

very different. In both hemispheres the temperature de- 
creases from equatorial toward polar regions, but the de- 
crease is much more rapid on some meridians than it is on 
others. 

This irregularity is caused by the different effects of 
heat upon land and water, owing to their differences (i) in 
specific capacities for heat, (2) in penetrability for heat 
rays, and (3) in state of aggregation, one being a solid 
and the other a liquid. 

Specific Heat. — It has been stated (page 23) that the 
same amount of energy produces different changes of 
temperature in different substances. Now, a water surface 
requires nearly twice as much energy to raise its tempera- 
ture by a given amount as an equal area of land ; that is, 
if equal surfaces of land and water, at the same tempera- 
ture, are equally exposed to the rays of the sun, the land 
will be warmed nearly twice as much as the water. 

Penetrability. — The solar rays cannot penetrate deeply 
into the solid land ; and, as land is a very poor heat 
conductor, all the sun heat received is confined to a 
thin surface stratum. This stratum is thus quickly and 
strongly warmed, and heats the overlying air in contact 
with it. But during the night, when the source of heat is 
withdrawn, the thin surface stratum of the land and its 
overlying air lose their heat by outward radiation with 
almost equal rapidity. Solar rays affect water, however, 
to a depth of about 500 feet, and warmth is thus distrib- 
uted throughout a comparatively thick layer, whose tem- 
perature, therefore, does not rise so high as that of the 
thin stratum of land. During the night the water surface 
cools more slowly than the land, for as soon as the surface 
becomes cooled in the slightest degree it contracts, be- 
comes heavier, and sinks, being replaced by the warmer 
and lighter water from beneath. 



HEAT OF THE ATMOSPHERE. 6 1 

The difference between the temperature of land and water is in- 
creased by the evaporation from the water surface, sensible heat of 
the water and overlying air becoming latent. Fully one half the 
sun heat falling upon the oceans is thus rendered insensible, while 
none of the heat falling on dry land becomes latent. Much of the 
heat rendered latent at the surface of the ocean is liberated over the 
land on the condensation of the vapor into clouds and rain, and 
thus warms the upper land air at the expense of the lower ocean air. 
Another reason why the sea is heated and cooled more slowly than 
the land, is that the ocean air generally contains more moisture 
than land air. The more moisture air contains the more impene- 
trable it is to solar rays (page 22). The air over the sea, therefore, 
stops and radiates back more of the entering sun heat during the 
day and more of the escaping surface heat during the night than 
does the drier land air. 

State of Aggregation. — The solid land is stationary, 
and retains or radiates its heat in the same place where it 
is received. Water, however, is susceptible of being- 
moved in currents, and thus of receiving heat in one 
place and losing it in another. Ocean currents carry warm 
water toward the poles and return cold water to the 
equator ; thus nearly one half the heat received by the 
torrid zone is conveyed into higher latitudes. 

Isothermal Charts. — If upon a map all places having 
the same mean temperature are connected by lines, such 
lines are called isothermal lines or simply isotherms. Taken 
collectively, these lines indicate the distribution of mean 
temperature over the region embraced in the map. Such 
a map is called an isothermal map or chart. On the ac- 
companying isothermal charts the regions in which the 
temperature is higher than jo° are tinted pink, those in 
which it is lower than 30 are tinted blue, while those 
having a temperature between 30 and yo° are untinted. 

Northern Hemisphere in Winter. — It will be seen on 
the chart that in temperate and polar regions the land air 
is colder than the sea air in corresponding latitudes, the 



62 PHYSICAL GEOGRAPHY. 

isotherm of 30 Fahr. descending to the neighborhood of 
40 latitude over the land, while over the sea this isotherm 
lies in much higher latitudes, for the land at this season 
loses more heat by radiation during the long nights than it 
receives during the short days, but the sea loses less heat 
by radiation and is constantly receiving heat by warm cur- 
rents from the equator. The difference in temperature is 
greater near the parallel of 6o° than in any other latitude, 
amounting to about 47 Fahr. In equatorial regions the 
land air is slightly warmer than the sea air ; thus, a tem- 
perature of more than 8o° Fahr. prevails over equatorial 
Africa and South America, while a temperature of less 
than 8o° Fahr. prevails over equatorial oceans, for currents 
carry heat away from the seas of these regions, while the 
diurnal loss and gain of heat are about equal on the land, 
since the nights and days are of nearly equal length 
throughout the year. 

Northern Hemisphere in Summer. — During the 
long days of summer the land receives more heat than 
it radiates during the short nights, and thus accumu- 
lating heat, becomes in all latitudes warmer than the sea 
surface in corresponding latitudes. The difference in tem- 
perature is not great, however,, because the sea absorbs 
heat by day as well as the land ; and besides, in higher 
latitudes, it is constantly receiving heat in warm currents 
from lower latitudes. The difference amounts to about 
1 8° Fahr. near the parallel of 6o°, and to but 25 Fahr. 
where it is greatest, — near the parallel of 40 . 

In the Southern Hemisphere, the distribution of tem- 
perature is seen to be much more regular than in the 
northern hemisphere, because its surface is more nearly 
uniform, being almost entirely water beyond 30 south 
latitude. The only considerable irregularity in the isotherms 
occurs in tropical regions, in the neighborhood of the 



64 PHYSICAL GEOGRAPHY. 

land surfaces. Here, as in corresponding regions in the 
northern hemisphere and for like reasons, the land is 
slightly warmer than the sea surface at all seasons. 

General Deduction. — It is thus seen that the water 
surface is not warmed so greatly during the day or during 
the summer, nor is it cooled so much at night or in winter 
as the land surfaces ; therefore, a water surface tends to 
preserve throughout the year a uniform temperature in its over- 
lying air, while the air over the land may become both ex- 
tremely hot and extremely cold. 

The Thermal Equator. — The line along which the 
greatest heat on the earth's surface occurs is called the 
thermal equator. As the sun is nearly vertical over the 
southern tropic in January, and over the northern tropic 
in July, it might be expected that during the year the 
thermal equator would travel backward and forward with 
the sun between these parallels. The different powers of 
land and water for accumulating and retaining heat, how- 
ever, greatly modify its annual journey. In July (page 63) 
the high temperature of the great land surfaces carries the 
thermal equator to between 20 and 30 north latitude 
over the continents, while, in consequence of the slower 
change of temperature of the water surfaces, it lies in the 
neighborhood of only io° north latitude over the oceans. 
In January, the summer of the southern hemisphere (page 
65), the influence of the larger land masses to the north is 
still great enough to hold the thermal equator very near 
the geographical equator in the southern hemisphere, 
while the western extensions of Africa and South America 
prevent it from crossing into the southern hemisphere at 
all in those regions. 



CHAPTER IV. 

MOISTURE OF THE ATMOSPHERE. 

All the rivers run into the sea ; yet the sea is not full : unto the place from 
•whence the rivers come, thither they return again. — Ecclesiastes i : 7. 

Source. — The atmosphere obtains its moisture by the 
process of evaporation from all the moist surfaces of the 
earth, but mostly from the great moist surface which 
covers three quarters of the globe — the sea. 

Vapor. — Water ceases to be a liquid upon evapora- 
tion, and enters the atmosphere as a gas called vapor. 
Vapor is transparent, and hence invisible. It is only after 
the vapor of the atmosphere condenses into a liquid (or 
solid) form that it becomes visible ; as vapor it can never 
be seen. 

Effect of Temperature. — A volume of air at a given 
temperature can hold only a certain quantity of vapor; 
if this air be warmed, it can hold a greater quantity of 
vapor ; if it be cooled, its capacity for vapor decreases. A 
cubic foot of air at a temperature of zero (Fahrenheit) can 
hold only half a grain of vapor; at a temperature of 3 2° 
it can hold more than 2 grains ; at a temperature of 6o° it 
can hold 5^ grains; at a temperature of 90 it can hold 
almost 15 grains, etc. 

Saturated Air. — When air at any given temperature 
contains all the vapor it can hold at that temperature, it is 
said to be saturated. If air which is not saturated comes 
in contact with a moist surface, it may evaporate water 
until it becomes saturated. If saturated air is cooled, it 

(66) 



MOISTURE OF THE ATMOSPHERE. 6j 

can no longer hold all of its vapor; a portion of it, there- 
fore, condenses into very small globules of water, or (if 
the temperature be low enough) into minute crystals of 
ice, and becomes visible. 

Effect of Evaporation and Condensation. — The im- 
mediate effect of evaporation is to make all bodies in the 
immediate vicinity colder, or to retard their growing 
warmer, sensible heat being abstracted from these bodies 
and converted into latent heat. Condensation warms sur- 
rounding bodies, or retards their cooling, since the latent 
heat again becomes sensible heat as the vapor passes into 
the liquid or solid form. 

General Distribution of Vapor. — Evaporation and 
condensation are constantly going on in nature, and there- 
fore the amount of vapor in the atmosphere is constantly 
changing ; but as warm air has a greater capacity for vapor 
than cold air, it is generally true that the amount of vapor 
in the air decreases from the surface of the earth upward, 
and from the equator toward the poles. It is estimated 
that almost one half the vapor in the atmosphere occurs 
lower than a height of one mile from the sea-level, and 
that fully nine tenths occur lower than four miles. 

Relative Humidity. — Since the capacity of air for 
vapor varies so rapidly with temperature, the absolute 
humidity, or amount of vapor present, gives no definite 
idea of the dampness of the air, for the amount of vapor 
which saturates air at 6o° temperature and makes it feel 
very damp, is but little more than one third of the 
amount required to saturate air at 90 temperature ; with 
this amount of vapor present, air at the latter temperature 
feels excessively dry and evaporates water with avidity. 
It is therefore common to determine the proportion which 
the vapor present at any temperature forms of the amount 
which would saturate the air at that temperature. This 

P. G.-s. 



6& PHYSICAL GEOGRAPHY. 

is called the relative humidity of the air. Thus, if the rel- 
ative humidity is 25^, 50%, or 7^%, the air contains y, 
y 2 , or Y^ of the vapor it is capable of holding at its tem- 
perature. Since air loses its capacity for vapor by cool- 
ing, it follows that when air is cooled its relative humidity 
increases, until, when cooled to the point of saturation, 
its relative humidity is 100. Any further cooling would 
produce condensation. Thus, since temperature decreases 
as the elevation above the earth's surface increases, evap- 
oration may be taking place in the lower part of a mass 
of air, while condensation is in progress in the upper part. 

The relative humidity is determined by means of an instrument 
called a Hygrometer. The hygrometer in most common use consists 
of two ordinary thermometers, the bulb of one of which is covered 
by a small piece of cloth kept constantly moist. The evaporation 
from this moist surface mak^s the bulb it covers colder than the 
other bulb, and the two thermometers register different temperatures. 
If the air is dry, evaporation is rapid and this difference is great ; if 
the air is moist, evaporation is slower and the difference in tempera- 
ture is less. Tables have been prepared from which the relative 
humidity corresponding to each degree of these differences between 
the " wet and the dry bulb thermometers," at any temperature, may 
at once be obtained. 

Mist or Fog is a vast multitude of minute globules 
of water in the air near the earth's surface. Fog may be 
produced by the spray thrown off by falling or otherwise 
violently agitated water, but it is usually caused by the 
cooling of saturated air, and the consequent condensation 
of a portion of its vapor. 

In winter, when our warm, moist breath passes from the mouth 
into the cold air, it is chilled, and a portion of its vapor condenses 
into a visible mist. Mists frequently form over sheets of water in 
summer nights, because the neighboring land cools at night faster 
than the water, and thus cools the atmosphere in contact with it ; 
this, in turn, chills the moist air over the water below its point 
of saturation, causing part of its vapor to condense into a mist. 
When the heat of the sun in the morning increases the capacity of 



MOISTURE OF THE ATMOSPHERE. 69 

the air for vapor, the mist evaporates and disappears. Mountain 
tops are frequently enveloped in mist because air currents, striking 
the mountain sides, are forced up the slopes into higher regions of 
the atmosphere, and thereby chilled below their point of saturation. 
The solid particles, or dust motes, in the air are great promoters of 
the formation of fogs, since they may radiate heat faster and thus 
become colder than the surrounding air, which is slightly cooled by 
contact with them. When this is the case, it is probable that each 
mist globule is formed around a dust mote. 

Clouds are merely fogs formed at some distance above 
the earth's surface. Clouds may be formed by radiation 
between warm and cold currents of air, but the chief 
causes of their formation are the mechanical cooling of an 
ascending current of air, and the cooling by radiation of a 
poleward- moving current of air. 

As air ascends and is relieved of a portion of atmospheric pres- 
sure, it expands, and pushes aside the surrounding air. In thus do- 
ing work, some of its energy must be expended ; that is, the velocity 
of its molecules is decreased, and it is cooled. Therefore, when air 
ascends it becomes constantly cooler. The reverse occurs when air 
descends; the air is compressed by the increased atmospheric pres- 
sure, and work is done upon it, whereby the velocity of its molecules 
is increased, and the air becomes warmer. Until its point of satura- 
tion is reached, ascending air is thus cooled i° for each 183 feet of 
ascent, but saturated air is cooled more slowly, owing to the effect of 
the liberation of latent heat on the condensation of its vapor. As 
descending air grows warmer its vapor does not condense, and there- 
fore both dry and moist air grow i° warmer for each 183 feet of descent. 

Height of Clouds. — Since most of the vapor occurs 
in the lower part of the atmosphere, clouds are most 
common at no considerable altitude. The mean elevation 
of clouds in the temperate zones is about one half a mile, 
while the highest clouds ever seen are probably within ten 
miles of the earth's surface. 

For convenience of description, clouds have been divided into 
three great classes, which are named from their general shapes: 



7o 



PHYSICAL GEOGRAPHY. 




*■ Cumulus . "K^ S Ira tics. 

Fig. 30. — Classes of Clouds. 



Nimbus. 



cirrus, or feathery clouds ; cumulus, or heaped-up clouds ; and 
stratus, or spread-out clouds. 

(1) Cirrus are the highest of all clouds. They are seen in fair 
weather as little, white, feathery patches in the blue sky. These 
clouds are so high that their temperature must be below the freezing 
point, and they are consequently thought to consist of minute ice 
crystals. 

(2) Cumulus are the familiar, dome-shaped masses of cloud hav- 
ing generally nearly horizontal bases. They are formed by ascend- 
ing currents of air, the horizontal base of the cloud marking the 
altitude where the decreasing temperature begins to condense the 
vapor of the ascending air. 

(3) Stratus are the continuous, horizontal layers of cloud, of 
general uniform thickness. They are the lowest clouds, and fre- 
quently appear in the morning and evening of fine days as a low, 
foggy canopy overspreading the whole or a part of the sky, and 
disappearing as the heat of the day increases. All low, detached 



MOISTURE OF THE ATMOSPHERE. J I 

clouds which look like lifted fog and are not consolidated into defi- 
nite form, are stratus clouds. 

By the various combinations of the three principal classes of 
clouds are obtained the cirro-stratus , or " Noah's ark " clouds ; cirro- 
cumulus, or "a mackerel sky"; cumulo-stratus, or rain-threatening 
clouds ; and nimbus, or the rain-cloud proper. 

Clouds are spoken of as suspended in the air, 
but their globules are generally descending slowly through 
the force of gravity. They generally do not descend far, 
however, before they reach warmer regions of the atmos- 
phere, where the lower portion evaporates and disappears. 
This accounts for the rapid change usually observed in the 
shape of clouds. Some portions of the cloud are disap- 
pearing by evaporation, while other parts are forming by 
condensation. 

One of the chief uses of clouds is the assistance 
they render in maintaining an equable temperature at the 
earth's surface. In its liquid form, moisture obstructs the 
passage of heat rays much more than in its vaporous 
form. Clouds, therefore, stop much of the sun's heat, 
and so prevent the earth from becoming too hot during 
the day-time ; while, by absorbing and radiating back a 
portion of the heat which is constantly streaming off from 
the earth, they prevent its surface from becoming too cold 
at night. This is the reason why cloudy days are gener- 
ally cooler, but cloudy nights warmer, than fair ones. 

Rain. — When a cloud is of considerable thickness, and 
the air beneath is nearly saturated, the globules in their 
gradual descent through the cloud unite to form larger 
drops, which, acquiring greater weight with their increase 
in size, descend faster than they evaporate; and, if the 
temperature be above the freezing point, may finally reach 
the earth as rain. 

Rain-water. — When water evaporates, all impurities are 
left behind ; vapor, therefore, condenses into absolutely 



72 PHYSICAL GEOGRAPHY. 

pure water ; but all the gases which compose the air are 
soluble in water, and hence rain-water, when it reaches the 
earth, is never pure, being always more or less impreg- 
nated with these gases, and containing besides dust motes 
and other solid particles which it has picked up in its de- 
scent through the air. 

Uses of Rain. — Rain, therefore, in addition to supply- 
ing the rivers, springs, and wells of the earth with water, 
performs an important office in washing and purifying the 
air and rendering it more healthful. 

Snow. — If the temperature of a cloud is below the freez- 
ing point, the cloud is composed of minute ice crystals 
instead of water globules. If the air beneath such a cloud 
is nearly saturated and of sufficiently low temperature, the 
ice crystals of the cloud accumulate in their descent into 
flakes, which may reach the earth. We call this phenom- 
enon snow. 

Shape of Snow-flakes. — When snow-flakes are formed 
in calm air, they arrange themselves, according to the 
laws of the crystallization of water, into little six-sided 
plates or six-pointed stars. Although over a thousand 
different shapes have been observed in snow crystals, each 
shape adheres to the general law of six-sidedness. 

Sleet. — When driven about by wind, the flakes lose 
this delicate arrangement, and when the temperature is 
such that the snow reaches the ground in a partly melted 
condition, it is called sleet. In the interior of continents 
the ground in winter is usually colder than the air, and 
the sleet upon reaching it immediately freezes, incasing 
the ground and vegetation in a coating of clear ice. This 
seldom happens on coasts and islands, where the moister 
air prevents the excessive cooling of the ground. In such 
localities, the continued melting of the sleet in the lower 
air gives it the appearance of a fine, driving rain. 



MOISTURE OF THE ATMOSPHERE. 



73 



Snow-storms are more frequent, and the snowfall is 
heavier when the temperature is near the freezing point 
than when it is much colder, because the colder air has so 
slight a capacity for vapor that it can yield but very little 
moisture to form snow. 

Snow Line. — Since the atmosphere grows colder with 
increase of elevation, there must be some altitude where 
the temperature seldom rises above the freezing point, 
even in summer. Above this altitude, the moisture of 




Fig. 31. — Some Shapes of Snow Crystals. 



the air is usually precipitated in the form of ice or snow, 

and if the precipitation is moderately heavy, the snow 

never entirely disappears from the ground. The lower 

limit of this region of perpetual snow is called the snow 

line. The snow line is higher in equatorial than in polar 

regions. 

The mean altitude of the snow line at the equator is about 16,000 ft. 



In the mountains of Spain, 37 N. lat. 



In the Swiss Alps, 
In Norway, . . 
In Lapland, . . 
In B'aren Island, 
In Spitzbergen, . 



47 N. lat., 
62 N. lat., 
6<f N. lat., 
75 N. lat., 



its altitude is about 11,000 
" " " 9,000 
" " " 5,000 

3>3°° 
600 



8o° N. lat,, it sinks nearly to sea-level. 



74 PHYSICAL GEOGRAPHY. 

Uses of Snow. — One of the chief uses of snow arises 
from the fact that it is a very poor conductor of heat. 
A layer of snow in winter acts as a blanket, preventing 
the loss of the earth's heat by radiation, and keeping the 
ground soft and moist; but the soil and vegetation left ex- 
posed lose their heat by radiation, and are frozen hard 
and stiff. 

Hail. — Pellets of ice falling in showers are called hail. 
These pellets, or hailstones, vary from the size of small 
shot to that of hens' eggs. Hailstones are sometimes 
composed throughout of clear ice, but usually there is a 
nucleus of hard, compact snow, surrounded by alternate 
layers of ice and snow. Hail is more common in summer 
than in winter, and in hot than in cool weather. It fre- 
quently precedes or accompanies a thunder shower. 

Hail is now believed to be caused by the rapid ascent and con- 
sequent rapid cooling of quite warm, moist air. Below a certain 
height the vapor condenses into cloud and rain, but above that 
height into snow. The rain drops carried aloft by the powerful 
current of air are frozen into clear hailstones of the ordinary size, 
which, upon being thrown outward beyond the influence of the as- 
cending current, fall to the earth. The snowy nucleus of other hail- 
stones is supposed to be formed as a minute snowball above the 
region of rain, and, in descending, to be several times drawn into 
the ascending current and repeatedly carried aloft before it reaches 
the earth, each time receiving a layer of ice in the region of rain, 
and of snow in the higher regions. 

Dew differs from fog or cloud in being little globules 
of water condensed from the atmospheric vapor, not in the 
air, but upon cool, solid bodies which have chilled the ad- 
jacent air below its point of saturation. 

If, in a warm room, a tumbler be filled with ice water, the outside 
of the glass will in a few minutes be clouded over with myriads of 
tiny water globules. These are in every way analogous to dew, and 
are caused by the chilling of the air adjacent to the cold glass, and 
the consequent condensation of its vapor upon the outside of the 



MOISTURE OF THE ATMOSPHERE. 75 

glass. The flitting cloud seen on a polished knife blade when 
breathed upon, is similarly caused by its momentarily chilling the 
warm breath and condensing part of its vapor. 

Natural Formation of Dew. — At night the earth 
radiates more heat than it receives, and becomes cooler. 
Clouds absorb and reflect back most of this heat, and so 
maintain the temperature of the earth throughout the 
night; but if the sky be clear, the temperature of surface 
objects may fall below the point of saturation of the ad- 
jacent air. When this happens, the excess of vapor in 
the thin layer of air next to the objects condenses into tiny 
water globules, which unite into dew-drops upon such cold 
surfaces as leaves and grass blades. As dew is thus 
formed as soon as the temperature of the air sinks below 
its point of saturation, the temperature of the point of 
saturation is frequently called the dew point. 

Hoar-frost. — If the dew point of the air be below the 
freezing point, the excess of vapor will be precipitated as 
fine spikelets of ice, which constitute hoar-frost. Hoar- 
frost is not frozen dew, but a sublimate, i. e., vapor pre- 
cipitated in a solid form. 

Both dew and hoar-frost are precipitated most copiously upon 
such objects as cool fastest, and thus become the coldest. Grass, 
trees, and herbage generally, though no better radiators than the 
soil or rocks, cool faster, because, being isolated, they lose heat by 
radiation faster than they receive it from below by conduction. 

Distribution and Amount of Precipitation. — The 

total amount of water precipitated upon the earth in all 
forms is for convenience called rain-fall. The amount of 
rain-fall received by the earth as a whole each year is about 
equal to the amount of water evaporated, but the amount 
of rain-fall received by the land is greater than the evap- 
oration from its surface, while evaporation is greater than 
rain-fall on the sea surface. On the average, the land 
loses by evaporation about three fourths of its rain-fall, 



MOISTURE OF THE ATMOSPHERE. JJ 

while about one fourth drains into the ocean, thus main- 
taining its level against the excess of evaporation from its 
surface. The rain-fall on the land amounts to about 30,000 
cubic miles of water annually, — enough to cover the 
whole land surface to a uniform depth of 33 inches. But 
all parts of the land do not receive equal quantities of 
rain-fall. The accompanying chart indicates the distribu- 
tion of mean annual rain-fall over the land. 

The reasons for the peculiar distribution will appear in the chapter 
on Climate, but it may be stated here (1) that the vapor taken up by 
the winds from the ocean is the ultimate source of rain-fall on the 
land; (2) that all sea winds reach the land nearly saturated with 
vapor ; (3) that such winds in warm latitudes contain much more 
vapor than in cold latitudes ; and (4) that the vapor in any wind is 
condensed into rain-fall only by the cooling of the air. This is usu- 
ally achieved either by the rising of the air or by its entrance into 
colder latitudes. 

Evaporation, like rain-fall, varies in different localities and in the 
same locality at different times. It is most active where the wind is 
strong and the air is relatively warm and dry, but it may cease alto- 
gether if the amount of vapor in the air is great. It is always 
active in any region when the wind is blowing from a colder region, 
or when the air is sinking, for in both cases the air is becoming 
warmer and its relative humidity is consequently decreasing. 



CHAPTER V. 

MOVEMENTS OF THE ATMOSPHERE. 

The wind goeth toward the south, and turneth about unto the north ; it whirl- 
eth about continually, and the wind returneth again according to his circuits. — 

ECCLESIASTES 1.6. 

Wind. — Sensible movements of air are called wind. 
Winds are caused by the force of gravity, - — the same force 
that causes the flow of rivers. Gravity, however, could 
not produce movement in the river had not moisture, in 
the form of vapor, first been raised to a higher level. 
This is accomplished by the energy of the sun's heat., 
The sun's heat also enables gravity to produce winds. 

Cause. — The sun makes some parts of the earth's sur- 
face warmer than others. The warmer part heats the air in 
contact with it. This air consequently expands. The expan- 

— sion may not affect the 

— highest layers of the at- 
-——-- ^l -- -^-----^ mosphere, but it pushes 

"711" the lower layers up into a 

A convexity over the Warm- 

er Earth's p Surface O QV regions, as ill Fig. 32. 

<MfwMM%BX ,,n '' Ilr,jUj !yi^MmM?M- Gravity now causes the 
Flg-32, movement indicated by 

the arrows, for, as the result of expansion below A, the 
air above is compressed and rendered denser than that 
over B. As part of the air over A thus moves away, the 
weight or pressure on D tends to decrease, and to increase 
over C ; but to equalize these pressures, the lower air 
moves as a wind toward D, The warm, expanded air is 
(78) 



MOVEMENTS OF THE ATMOSPHERE. 



79 




lighter than the surrounding cool air, and is forced by it 
to rise, thus forming an ascending current over the warm 

region, while over the 

surrounding cooler re- 
gion the air is gradually 
settling downward as 
the bottom air moves 
from under it. The 
general movements in- 
dicated by the arrows Flg - 33> 
in Fig. 33, result. As the air thus rises over but one lo- 
cality, while it sinks down in the entire surrounding 
region, it must ascend faster than it descends. 

The movements continue as long as the central air is warmer 
than that surrounding, for so long the densities are unequal, and 
gravity produces movement. Thus, every wind that blows on the 
earth's surface has its counterpart, blowing in a different direction,' 
at some distance above that surface. The lower wind blows toward 
a region of low pressure, where the air is rare and rising, and from 

a region of high pressure, 
where the air is dense and 
sinking. 

The Rotation of the 
Earth appears to deflect 
all winds from a straight 
course. The deflection 
is to the right in the 
northern, but to the left 
in the southern hemi- 
sphere. 

Suppose figure 34 to rep- 
resent the northern hemi- 
sphere, and a wind, shown 




Fig. 34- 



by the arrow at a, to be blowing poleward along the meridian A. 
While the wind is advancing to b, c, and d, the rotation of the earth 
carries meridian A forward, say to the positions of B, C, and D re- 



8o 



PHYSICAL GEOGRAPHY. 



spectively. The change in the direction of the meridian, conse- 
quent upon its change of position, causes the direction of the wind, 
which was northward at a, to become successively more and more 
easterly at b, c, and d ; and as we are apt to regard the direction of 
the meridian as fixed, an apparent deflection of the wind to the right 
is the result. The same cause produces a gradual northward deflec- 
tion of a wind blowing due west over meridian E, as rotation carries 
the meridian of the wind successively to positions F, G, H. A wind 
blowing south on meridian / appears to turn westward, as rotation 
carries its meridian to positions J, K, L ; while a wind blowing due 
east on M appears to turn southward as its meridian advances to 
N, O, P. Thus, a wind blowing in any direction in the northern 
hemisphere appears to turn to the right from its original course as it 
advances. In the southern hemisphere, the apparent deflection is 
to the left, because when we change our point of observation from 
the north to the south pole, the direction of the earth's rotation ap- 
pears to be reversed. Figure 34 accurately illustrates the cause of 
the apparent deflection, but exaggerates its amount. Really there is 
no deflection of winds at the equator ; but 
on leaving the equator, the amount of the 
deflection increases first rapidly, and 
then very slowly, and is greatest near the 
poles. 

The Effect of this Deflective 
Tendency is to prevent the winds 
from moving directly toward a warm 
region. Starting directly toward it, 
the winds are deflected as they ad- 
vance, and so approach the warm 
region obliquely ; hence, when winds 
from all directions blow toward some 
central area, the deflective influence 
causes them to form a spiral whirl 
around the central area. The direc- 
tion of the whirl is obviously to the 
left of an observer at its center in 
the northern hemisphere, but to the right in the southern 
hemisphere. 




IN NORTHERN 1 HEMISPHERE 
IN SOUTHERN HEMISPHERE 




Fig. 35- 



MOVEMENTS OF THE ATMOSPHERE. 51 

In approaching a central point, the winds move as if confined 
in constantly narrowing paths, and hence blow with increasing 
violence as they advance. This is because the air that crosses the 
broader portion of the path, near the margin of the whirl, must cross 
the narrow portion, near its center, in equal times, in order to make 
room for the following air. 

Pressure in the Whirl. — When water has a rapid 
rotary motion, as in an eddy, its surface is observed to be 
depressed near the center and elevated near the circumfer- 
ence of the whirl. This is caused by the centrifugal force 
developed by its rotary motion. The same force is devel- 
oped by the rotary motion of air, and causes a decrease 
of atmospheric pressure in the center of a whirl, from 
which the pressure increases gradually to its circumfer- 
ence. 

The lowest layer of air, being greatly impeded by friction on the 
earth's surface, does not rotate so fast as the next higher layer, and 
each layer, being less dense, offers less frictional resistance to the 
stratum above ; hence, the upper strata develop great centrifugal 
force and a large central area of depression, while the lower strata 
develop less centrifugal force and a small area of depression. The 
lower winds, pushed by the greater pressure behind, flow spirally 
toward the central area, where they slowly ascend ; they move fast- 
est near the center, and as they flow spirally outward aloft, their 
velocity decreases. 

Three Classes of Winds, — Since winds are caused 
by inequalities in the weight or density of the atmos- 
phere in adjacent regions, and since these inequalities of 
weight are caused primarily by differences of temperature, 
winds may be divided into three classes according to the 
permanence of their exciting cause: (i) As equatorial re- 
gions are always warmer than polar regions, there must be 
winds constantly blowing toward the equator in the lower 
atmosphere, and from the equator in the higher atmos- 
phere. These may be called Constant winds. (2) As the 
land and water surfaces have different temperatures, the 



82 PHYSICAL GEOGRAPHY. 

land being generally warmer in summer, and the water in 
winter, there must be winds blowing, in the lower atmos- 
phere, toward the land during one part of the year, and 
from the land during another part of the year. These may 
be called Periodic winds. (3) The whole of a land or water 
surface is seldom equally heated; some places are hotter 
than others, owing to local or temporary causes. There 
are, therefore, temporary winds blowing in the lower at- 
mosphere toward these warmer places from all surround- 
ing regions. These may be called Occasional winds. 

Constant Winds. — The high temperature near the 
equator creates a belt of expanded and rising air, toward 
which surface winds blow from the northern and southern 
hemispheres, and from which the upper winds move over 
either hemisphere. The movements cause a belt of low 
pressure along the thermal equator, and since the upper 
winds, moving from the equator, are advancing from all 
directions toward a common center (the pole), they gradu- 
ally form an immense whirl, which in turn causes an area 
of low pressure near either pole. 

Tropical Belts of High Pressure. — Between the 
equatorial belt of low pressure, caused primarily by heat, 
and the polar low pressure, caused directly by the whirl 
of the winds, there must be, in either hemisphere, a belt 
of relatively high pressure. The mean position of this 
belt is in the neighborhood of that parallel which divides 
the surface of either polar hemisphere into two equal 
parts — the parallel of 30 N. or S. latitude. It has been 
seen that the lower air is pushed out in all directions from 
under an area of high pressure toward areas of lower pres- 
sure. Consequently, surface winds issue from the tropical 
belts of high pressure toward the equatorial low pressure 
belt on one side, and toward the areas of polar low pressure 
on the other side. 



MOVEMENTS OF THE ATMOSPHERE. 83 

The Trade Winds. — On the equatorial side the winds 
advance readily, being urged forward by the high pressure 
into regions where the air is warmer and lighter. Conse- 
quently, they blow with great steadiness throughout the 
year. They are gradually deflected to the westward by 
the earth's rotation, and, since winds are named by the 
direction from which they blow, they become north-east 
winds on the northern and south-east winds on the south- 
ern side of the thermal equator. Their uniformity in force 
and direction won for them the name trade winds, because, 
like trade, they follow a fixed or trodden path. Their 
mean velocity is about 6^2 miles an hour. 

The Antitrade Winds. — The surface winds which 
issue from the polar sides of the tropical belts of high 
pressure, are urged forward by that pressure into regions 
where the air cools and becomes heavier. This frequently 
impedes the advance of the air, and consequently the 
winds are not so constant as the trade winds. By the 
earth's rotation they become south-west winds in the 
northern, and north-west winds in the southern hemisphere ; 
and since these directions are opposite to those of the trade 
winds, these winds are called the antitrade winds. Since 
these winds form part of the great polar whirl, their ve- 
locity increases as they "approach the center of the whirl 
(page 81). Their mean velocity on the Atlantic in 50 
latitude is about 30 miles an hour. 

Belts of Calms. — In the belt of low pressure near the 
thermal equator, where the motion of the air is upward, 
and in the tropical belts of high pressure where the motion 
of the air is downward, the movement is largely insensi- 
ble, and calms or light, variable winds are the result. 
These calm belts travel northward and southward as the 
sun becomes vertical over different latitudes in different 
seasons of the year. In the Pacific, the equatorial calms 



84 PHYSICAL GEOGRAPHY. 

extend south of the equator in January, but lie entirely 
north of it in July; while over the greater part of the 
Atlantic they are north of the equator during the entire 
year — about 2° north in January, and about io° north in 
July. The trade and antitrade winds and the belts of 
calms are better defined on the ocean than on the conti- 
nents, because the sea surface has a more uniform temper- 
ature, and because that surface is smoother than the land 
and offers less frictional resistance to the winds. 

The constant winds are more plainly marked in the southern 
hemisphere than in the northern hemisphere, because there is but 
little land in the south temperate zone to become in turn hotter and 
colder than the surrounding ocean as the seasons change, and thus 
to modify the direction of the winds. Therefore, the Wind Charts 
of the Southern Hemisphere, on the opposite page, are given first. 
On these is shown the direction of the prevailing winds in summer 
(January) and in winter (July). Isobars, or lines drawn through 
places where the atmospheric pressure is the same, are also shown, 
the isobars denoting the mean or a high pressure (30 inches or over) 
being drawn in red, while isobars of low pressure (less than 30 
inches) are drawn in blue. It is seen that a belt of high pressure 
lies over each of the oceans in about 30 latitude in January, while 
in July this belt of high pressure almost encircles the hemisphere 
in this latitude. The winds blow obliquely out from these regions 
of high pressure, forming the trade winds on the equatorial side, 
and the antitrade winds on the polar side. 

Periodic Winds. — In the neighborhood of the conti- 
nents the direction of the trade and antitrade winds is con- 
stantly undergoing a gradual change, owing to the seasonal 
variation in the relative temperature of the land and water; 
hence, in such regions the constant winds become periodic 
winds. There are two kinds of periodic winds : seasonal 
winds and diurnal winds. 

Monsoons. — Most of the land on the globe lies in the 
north temperate zone. In these latitudes, it has been seen, 
the land js warmer than the adjacent ocean in summer, but 
colder than the ocean in winter. In summer, therefore, 



86 PHYSICAL GEOGRAPHY. 

the air over the land is the more expanded, and forms a 
region of relatively light air, toward which the surface 
winds blow from the surrounding oceans, and in which 
they escape by ascending to the upper atmosphere. 

In winter, on the contrary, the warmer oceanic air is the 
more expanded, and the colder land air is relatively dry 
and dense. The surface winds at this season consequently 
blow outward in all directions toward the ocean from the 
land region of greatest density where the air is sinking. 
These winds, blowing toward the land in summer and from 
the land in winter, are called monsoons, from an Arabic 
word meaning season. 

Since uniformity of temperature is more disturbed by a large 
land surface than by a small one, the monsoons of continents are 
stronger and steadier than those of islands, and those of large con- 
tinents than those of small continents. Since vapor obstructs the 
passage of heat rays, and since the amount of vapor in the atmos- 
phere decreases upward very rapidly, the surface of high land is 
heated in summer, and is cooled (by radiation) in winter much more 
readily than low land with its moister atmosphere. Hence, a conti- 
nent composed of highlands will have much stronger monsoons 
than a low continent. The great influence of the extensive land 
masses in the north temperate zone in modifying the direction of the 
winds in their neighborhood at different seasons is well shown on 
the Wind Charts of the Northern Hemisphere. Thus, in southern 
and eastern Asia, and over the eastern and western portions of 
North America, the direction of the winds in January is almost oppo- 
site to that prevailing in July. 

The Monsoons of Asia and Australia. — Owing to 
the peculiar position of Asia with relation to the Indian 
Ocean, to its vast extent, and to the occurrence in that 
grand division of the most extensive region of very high 
land on the globe (the plateau of Thibet), the monsoons of 
the northern Indian Ocean and the Malay Archipelago are 
particularly well marked. 

In summer, the heat upon the Asiatic highlands is greater, and the 
air is less dense than that on the equatorial Indian Ocean, and the 




HEMISPHERE 

f Isobars every j^'i 1 inch. 
Arrows fly with 
the winds. 



88 PHYSICAL GEOGRAPHY. 

southern trade winds of that ocean sweep north of the equator. 
Here, influenced by the earth's rotation, they veer to the right and 
reach the coast of Arabia and India as the south-west monsoon. 
This monsoon blows steadily from May to October. Southerly and 
easterly monsoon winds prevail at this season on the south-east and 
east coasts of Asia, northerly winds in the northern part of the 
grand division, and north-westerly and westerly winds blow over 
Europe. 

In winter, all this is changed. At that season Asia is colder than 
the adjacent oceans, and the air over it becomes very dry and dense. 
The winds blowing from this region of very dense air from October 
to May are influenced by the earth's rotation, and become the 
steady north-east monsoon of the north Indian Ocean, the north-west 
monsoon of the east coasts of Asia, southerly winds in Siberia, and 
easterly or south-easterly winds in eastern Europe. 

The Monsoons of the other Grand Divisions are 

similar but not so pronounced as those of Asia, owing to 
their smaller size and lower surface. The North Amer- 
ican regions of low pressure in summer and high pressure 
in winter are quite perceptible, however, and the latter, in 
connection with the high pressure existing over Asia at 
that season, has a marked influence upon the winter winds 
of the intervening oceans. 

Effect on Winds of North Atlantic and Pacific 
oceans. — Since the eastern and western continents nearly 
touch at Bering Strait, the very dense air lying over the 
continents in winter, and that composing the belt of per- 
manent high pressure over the tropical oceans, quite sur- 
round the northern parts of the Atlantic and Pacific 
respectively, over each of which the air is rare and the 
pressure low. The surface winds flowing from all sides 
into these regions of lighter air, are deflected by the 
earth's rotation into a great whirl over each of the oceans. 
The center of these whirls is near the parallel of 6o°, in 
which latitude the difference between continental and 
oceanic temperatures is greatest (page 87, January). 



MOVEMENTS OF THE ATMOSPHERE. 89 

Diurnal Winds of Coasts. — Near the coast the land 
air has nearly the same mean temperature as the adjacent 
sea air; but since it rests on a land surface, the air be- 
comes slightly warmer during the day and slightly cooler 
at night than the sea air. A sea breeze consequently 
springs up during the forenoon and blows inland until 
night-fall, when, after a short calm, a land breeze begins 
to blow toward the sea, and continues until morning. On 
tropical coasts, these breezes occur regularly throughout 
the year ; in higher latitudes they are not noticeable in 
winter, the land air being so chilled by radiation during 
the long nights of that season that it fails to attain the 
temperature of the sea air during the short days. 

Diurnal Winds of Mountain Valleys. — Since the 
earth's surface is quickly heated by the sun's rays by day, 
and quickly cooled by radiation at night, and since this 
relatively hot or cold surface largely governs the tempera- 
ture of the air resting upon it, it follows that the air rest- 
ing on highlands may become hotter by day than the air 
at the same altitude over adjacent valleys or lowlands. 
When thus heated and expanded, the highland air flows 
off above and increases the pressure in the valleys. The 
increase of pressure drives surface winds up the valleys by 
day. At night the highland ground and the air resting 
on it may become cooler than the air at the same altitude 
over the lowland. It contracts in cooling, and upper cur- 
rents begin to move toward it, thus tending to increase 
the pressure on the highland, and drive surface winds 
doivn the valleys by night. 



P. G.— 6. 



CHAPTER VI. 

MOVEMENTS OF THE ATMOSPHERE Continued. 

Occasional Winds include all winds which are usually 
called storms. They also include many winds which are 
similar to storm winds, but are not so violent. They may 
occur in any latitude, but are very much more frequent 
between the parallels of 40 and jo° than they are in 
other latitudes. 

Whirling Motion. — Since occasional winds are directly 
caused by the difference in density between a compara- 
tively small central region where the density is relatively 
slight, and the surrounding regions, where the density is 
relatively great, the air of the surrounding regions, in 
moving toward the central region, gradually acquires a 
whirling or rotary motion, which is characteristic of all 
occasional winds. 

The whirl may begin as a comparatively small affair, sometimes but 
a few miles in diameter ; but by the decrease of pressure in the central 
area, owing to the centrifugal force of the rotating winds, the diame- 
ter of the whirl may increase to 2,000 miles. The force of the winds 
which constitute the whirl gradually increases from the margin 
toward the center of the whirl. Thus, it may be a gentle breeze 
near the margin, and be blowing with the hurricane force of 100 
miles or more an hour near the center. 

Progressive Movement. — In addition to the whirling 
motion, occasional winds have also a progressive move- 
ment ; that is, the center of the whirl, instead of remain- 
ing stationary, moves from place to place. Neither the 

direction nor the speed of this motion is regular, but it is 

(90) 



92 PHYSICAL GEOGRAPHY. 

generally in nearly the direction of the prevailing wind 
in which the whirl occurs. The general movement is 
westward and away from the equator in the torrid zone, 
but eastward and away from the equator in the temperate 
zones. The average speed is from 8 to 14 miles an hour 
in the torrid zone, but from 17 to 28 miles an hour in 
higher latitudes. 

These Moving Whirls constitute occasional winds. 
The direction of the wind at any place depends upon the 
position of the center of the whirl 
with relation to the place at the time. 
Thus, suppose A and B (Fig. 36) 
to be two places 500 or more miles 
apart, but both lying in the anti- 
trade wind region of the northern 
hemisphere. Let the long arrow, 
represented as flying north of east, 
indicate the general direction of the 
antitrade wind, and the direction in which a great whirl, 
represented by the small arrows, is progressing. It will 
be seen that, as the whirl passes, the wind at A is first 
south-east, then east, and finally north-east; while at B 
the wind is first south-west, then west, and finally north- 
west. Occasional winds may be broadly divided into 
three classes: (1) dust whirlwinds, (2) ey clones, and (3) 
tornadoes. 

Dust Whirlwinds are essentially the draining away, 
upward, of a thin layer of calm, dry air, which has be- 
come excessively heated by contact with the sun-warmed 
earth. As sunshine is required to heat the earth, these 
winds occur only in the day-time. Once formed, they 
continue until the layer of heated air has drained away or 
been cooled by contact with the cooling earth after sun- 
set. As vegetation affords a protection against the sun's 




MOVEMENTS OF THE ATMOSPHERE. 93 

heat, dust whirlwinds are most frequent over deserts or 
hot turnpike roads. Although strong enough to carry 
along dust, sand, straw, and leaves, these whirlwinds sel- 
dom attain a disastrous force because of their short dura- 
tion and consequent small diameter, and also because of 
the friction with the earth's surface of the thin stratum of 
air in motion. 

In the intensely hot and sandy deserts of tropical regions, as well 
as in Arizona and other parts of the West, these whirlwinds attain 
their greatest development. In Africa and Arabia they are known 
as simooms, and are dreaded not only for the heat of the wind, but 
for the immense clouds of sand with which they fill the air. 

Cyclones differ essentially from dust whirlwinds in be- 
ing composed of moist instead of dry air. Several im- 
portant peculiarities result from this difference. Moist air 
is heated directly by the sun's rays in the day-time, and 
its cooling is retarded by the radiation from the earth at 
night ; hence, if the air is calm, heat may accumulate, 
and a much thicker layer of air may become excessively 
warm before it begins to drain upward than in the case 
of the dust whirlwind. When movement begins, where 
the air is most expanded and moist, the air cools as it 
ascends, and a part of its vapor condenses into clouds ; 
hence, rain generally accompanies a cyclone. The con- 
densation of the vapor liberates latent heat, which pre- 
vents the ascending moist air from cooling rapidly. From 
all these causes it results that a cyclone is not a mere day- 
time whirl like the dust whirlwind. It generally continues 
for several days, and may grow from the effects of cen- 
trifugal force, so as to involve in the whirl all the air 
within 1,000 miles or more of the center. 

A cyclone generally continues and increases in size 
until the air, ascending in the central column, is no longer 
able to flow off above, beyond the outer margin of the 



94 PHYSICAL GEOGRAPHY. 

cyclone. Then, as more air flows in below than flows out 
above, the rotation slowly ceases, the pressure rises at the 
center, and the cyclone dies away. The progression of a 
cyclone frequently carries it half-way round the earth be- 
fore it dies away, as shown by the chart on page 91. 

In tropical regions, cyclones are most violent because the heat 
and moisture are greatest. They are called hurricanes in the trop- 
ical Atlantic, and typhoons in the Pacific and Indian oceans. Since 
cyclones require calm air for their formation, they originate in the 
torrid zone in general only in the equatorial calms; but as their de- 
velopment depends upon the deflective influence of the earth's 
rotation, which decreases to nothing at the equator, hurricanes are 
most frequent in summer, when the equatorial calms are most dis- 
tant from the equator, and they rarely occur in the south Atlantic 
because these calms never extend far south of the equator. In the 
south Indian and Pacific oceans, also, typhoons are most common 
in summer (February). In the north Indian Ocean, typhoons are 
most frequent at the change of the monsoon, at which season light, 
variable winds and calms prevail. Since these storms require some 
time to grow, and as they are traveling westward and away from the 
equator while developing, it is the western part of the tropical 
oceans in which cyclones are most frequent and most violent. The 
cyclones of temperate zones are less violent than tropical cyclones, 
because the atmospheric heat and vapor are less in these latitudes ; 
but they are much more numerous, because the deflective influence 
of the earth's rotation is so much greater that local areas of low 
pressure are more apt to grow into great cyclones. These areas of 
low pressure occur in great numbers in temperate latitudes as a 
result of the cooling of the antitrade winds as they advance into 
higher latitudes, and the consequent formation of local calms and 
areas of relatively low pressure. Because of the great numbers of 
these that develop into cyclones or storms, the regions between 40 
and 70° latitude are the great storm zones of the world. This is 
particularly true north of the equator, where the irregularities of 
the greater land surface, by directing the air upward, promote the 
formation of cyclones. 

Rain Area in a Cyclone. — In the temperate zones 
most of the rain accompanying a cyclone falls east of its 
center, for it is on this side that the winds of the whirl 



MOVEMENTS OF THE ATMOSPHERE. 95 

move into regions where they become colder, and in con- 
sequence of this much of the vapor is condensed. The 
greater amount of latent heat thus liberated on the eastern 
side of a cyclone is one of the prime causes of its pro- 
gressive eastward movement in the temperate zones. 

Anticyclones. — Many cyclones exist simultaneously in 
the temperate regions. They follow each other in rapid 
succession. One may overtake another, and they may 
coalesce into but one larger cyclone ; or, owing to local 
peculiarities of heat and moisture, a large cyclone may 
divide into two, which diverge and 
pursue slightly different courses. The f f * ^ 

region between the margins of ad- v \ \ + /^_J~^ 
jacent cyclones is of course a region ^VV. H^ u ,e 

of relatively high pressure, and is 



called an anticyclone, because it dif- ^^ * \ \ 

fers in almost every respect from a / / j * 

cyclone. Thus, (a) being a region of in northern hemisphere 

relatively high pressure, the surface IN southern hemisphere 

winds blow in all directions out from ^- \ \ I 

it (Fig. 37) instead of into it; (d) T~^ \ t / / 

these winds are deflected by the ^ ., -%■/ ^r 

earth s rotation into outward mov- , 1 I \ e v^ 

ing spirals, the whirl being to the 1 \ \ ^^ " 

right of an observer at the center, / \ x 
in the northern hemisphere, and to 

. - 1 Fig. 37. — Anticyclones. 

the left in the southern, and conse- 
quently in a contrary direction to the movement in a 
cyclone ; and (c) as the air descends in regions of rela- 
tively high pressure it becomes warmer, and therefore its 
vapor does not condense ; hence, anticyclonic winds do 
not usually bring cloudy or rainy weather. Traveling to 
regions of greater differences of pressure, these winds 
move faster as they advance. 



9 6 



PHYSICAL GEOGRAPHY. 



Tornadoes. — A tornado is a whirl of small diameter, 
but great depth and velocity, which forms a short distance 
above the earth's surface, and into which the surface air 
is "sucked up" with excessive violence. It is believed 
that tornadoes constitute a small secondary whirl within 
some gently moving cyclone. They have been known to 
form at all hours and in all seasons, but they occur most 
frequently during sultry afternoons of summer. They are 






Fig. 38 



-A Tornado. 



supposed to be directly caused by a warm, moist current 
of air at some distance above the earth's surface, but 
underneath a higher current of cold, dry air. The moist- 
ure in the warm layer accumulates heat directly from the 
sun's rays and by the radiation from the earth, and be- 
comes excessively hot in comparison with the air above. 
The secondary whirl, or tornado, forms about some point 
where the thick layer of hot air begins to escape upward, 
and rotates in the same direction as the cyclone in which 
it is formed. As the moist air ascends, expands, and 



movements of the atmosphere. 97 

cools, the vapor condenses and forms the gyrating, funnel- 
shaped cloud, hanging small end downward, which serves 
as a warning of the tornado's approach. The funnel is 
formed some distance above the earth's surface, in the air; 
and as friction is there very slight, the winds of the whirl 
attain enormous speed, and develop great centrifugal force, 
which causes a decided decrease of pressure in the funnel. 
The surface air being thus suddenly relieved of a great 
portion of the weight of the air above, expands, often with 
explosive violence, and rushes with great rapidity up the 
funnel. Violent surface winds rush in from all sides to 
take its place and follow it upward. These surface winds 
constitute the destructive blast of the tornado. 

The force of the tornado blast is terrific ; it blows down the 
strongest houses and largest trees, and carries such heavy objects as 
carts, iron chains, beams, and even men and women whirling aloft 
in the gyrating funnel. To produce such results, a wind velocity of 
over 200 miles an hour is thought to be necessary. The tornado 
winds, however, seldom attain destructive violence over a track ex- 
ceeding one fourth mile wide. Like the cyclone, a tornado has a 
progressive motion, usually in the direction of the prevailing winds 
of the region where it occurs. In the United States, tornadoes 
usually travel north-east at a speed of about 30 miles an hour. The 
tornado continues until the layer of hot air has drained away ; this 
usually takes about an hour ; hence, the track of a tornado is usu- 
ally about thirty miles long. 

Thunder-storms with rain and hail usually accom- 
pany tornadoes. The vapor of the ascending and expand- 
ing air in the funnel is condensed into water, which, 
carried upward by the powerful updraught, is converted 
into hail. The friction of the rapidly ascending air and 
water particles possibly generates the electricity mani- 
fested in the thunder-storms. 

Cloud Bursts. — The ascent of air in a tornado may be 
so violent as to prevent the fall of rain-drops, and thus 
cause an enormous accumulation of water in the air. 



MOVEMENTS OF THE ATMOSPHERE. 99 

Upon the cessation of the updraught, the water may fall 
in continuous streams. This is called a cloud -biti'st. Each 
of these streams may excavate a great hole, or basin, in 
the ground, and on steep slopes may occasion a land-slide 
or a great ravine, and wash large rocks and trees bodily 
down the hillside. Such a cloud burst occurred at Spring- 
field, Ohio, in May of 1886, and occasioned great dam- 
age, inundating dwellings, and washing away railroad 
embankments. 

Water-spouts and White Squalls. — When a tornado 
occurs at sea, and its funnel-shaped cloud descends to the 
surface of the water, the violently agitated water is sucked 
up for a short distance into the funnel; but at a height of 
a few feet it breaks into spray, which is carried aloft by 
the whirling winds, thus presenting the appearance of a 
solid, whirling column of water, or water-spout. White 
squalls are simply small, fair weather tornadoes. They fre- 
quently cause water-spoUts, and may be quite violent. 

Frequency of Tornadoes. — During the twelve years 
prior to 1883, over 500 tornadoes occurred in the United 
States, or an average of one every nine days. They occur 
in Kansas, Illinois, and Missouri more frequently than 
elsewhere in the Union. In this region the warm south- 
erly surface winds of summer, underrunning the colder 
westerly winds in the upper atmosphere which have 
crossed over the Rocky Mountains, afford conditions pecu- 
liarly favorable for tornado formation. The accompany- 
ing chart shows the relative tornado frequency in different 
parts of the United States. The deeper shading shows 
regions where tornadoes are more frequent. 



CHAPTER VII. 

LUMINOUS PHENOMENA OF THE ATMOSPHERE. 

By what way is the light parted ? — Job xxxviii : 24. 

Apparent Displacement of Heavenly Bodies by 
Refraction. — The sun, moon, and other heavenly bodies 
are visible when they are really below the horizon, owing 
to the refraction (page 26) of their rays as they penetrate 
the increasingly denser atmosphere in approaching the 
earth's surface. In the torrid and temperate zones the sun 
thus appears to rise in the morning earlier and set in the 
evening later than it really does by from 2 to 27 minutes, 

^ according to the latitude 

-•"' a j\_ Cv\ of the observer and the 

;<||§| /^\ e '' E \ ' — ~(3 B season of the year. 

i If AB represent the hori- 

w / zon line of an observer at 

A, a ray from the center of 
Fig. 39. J 

the sun or moon, C, entering 

the atmosphere at E, is refracted into a curve as it traverses succes- 
sively denser layers of air. The observer at A sees the sun in the 
direction in which the ray is traveling at the instant it enters his eye, 
or at D. The amount of this displacement decreases from about 
the apparent width of the sun, at the horizon, to nothing at the 
zenith, where the rays fall with no obliquity. This explains the oval 
shape sometimes observed in the sun or moon when near the hori- 
zon; for the lower edge, whose rays strike the atmosphere more 
obliquely, is displaced more than the upper side. 

The Sun and Moon appear larger near the Hori- 
zon than when higher in the sky. They are, of course, 
(100) 



LUMINOUS PHENOMENA. 



IOI 



not really any larger or nearer at such times, and the ap- 
pearance has nothing to do with refraction, but arises 
simply from a common error of judgment on the part of 
the observer. 

All objects appear smaller as their distance increases, and our 
only means of judging the size of a distant object whose dimensions 
are unknown, is by comparing it with the apparent size of some 
equally distant but familiar object — as a man, a tree, a house, etc. 
When low in the sky, the sun or moon is in a position where it may 
be directly compared with familiar objects on the distant horizon, 
and its great relative size impresses itself upon us and makes it seem 
actually larger than when seen higher in the heavens with no 
standard of comparison near it. For the same reason we are apt to 
think that a full grown man at the top of a steeple 200 feet high is 
only a boy. No one makes such a mistake with regard to a man at 
the same distance when he is surrounded by familiar objects on the 
earth's surface which serve as standards for comparison. 

Twilight. — After the sun has disappeared below the 
horizon, the earth is not immediately plunged into dark- 
ness : objects remain visible by the light reflected from the 
higher parts of the atmosphere which is still traversed by 
sunbeams. This is called twilight (half light). The same 
phenomenon occurs before sunrise, and is called dawn. 

Even when the sun is in the zenith of a cloudless sky, as much 
as one fifth of the light we receive is that which is reflected to us 
from other quarters of the sky than that through which the beams 
penetrate directly to us. When the sun is just above the horizon, 
more than two thirds of our 
light is that which is re- 
flected from the sky; and 
when it is invisible below 
the horizon, all our light is 
so reflected to us. Suppose 
the ray SE (Fig. 40) is the 
last from the setting sun which strikes A. A is illuminated by this 
direct ray, and by rays reflected from every point of the sky from H 
to F which is still traversed by the sun's beams. The sun's rays can 
not reach A when rotation has carried it to C, and lifted its horizon 




TWILIGHT 

Fig. 40. 



FULL LIGHT 



102 PHYSICAL GEOGRAPHY. 

to G; but Cis not dark because reflected light from every point of 
the sky between F and G reaches and illuminates it with twilight. 
When the horizon is lifted to F, however, by the earth's rotation to 
D, neither direct nor reflected light from the sun reaches that part 
of the earth's surface, and darkness prevails. 

"The Sun drawing up Water." — When a ray of 
light is admitted into a dark room, its path becomes visi- 
ble by the light reflected from the air particles and float- 
ing dust motes (page 27). The same phenomenon on a 
grand scale is sometimes seen in the open air when the sun- 
beams, breaking through rifts in the clouds, are rendered 
visible in the clouds' shadow by the light reflected to the 
eye from the strongly illuminated dust and air particles in 
their path. This phenomenon is frequently but errone- 
ously supposed to be "the sun drawing up water." 

Mirage. — Adjacent layers of air near the earth's sur- 
face have sometimes, owing to differences in temperature 
or humidity, widely different densities. The refraction 
and total reflection of light rays in traversing such layers 
often give rise to distorted, displaced, or inverted images 
of the objects from which they proceed. This phenom- 
enon is called mirage. The suitable atmospheric condi- 
tions may occur in any region, but are probably most 
.^^w^ frequent over hot deserts. 
^^T* Looming and Fata Morgana 

, /^U are but peculiar instances 

#^ f£^ —>^- <£^* °f mirage. 

iU -~-* > s -■■*--■- ~^-rr^~-— -=-"%~~ _E;. Suppose the heated ground has 

' ""--c j/ warmed the lower layers of the 

\^|^L? atmosphere, A, B, C (Fig. 41), to 

-*' ;// a much higher temperature, and 

thereby made it much rarer than 

the air above. The rays indicated by the dotted line reveal the 

tree to the observer at F in its proper position, but the ray striking 

the layer of rare air at E is refracted more and more as it enters 

rarer layers, until it strikes a layer so obliquely as to be totally re- 



LUMINOUS PHENOMENA. 



IO3 



fleeted at D. The observer sees an inverted tree in the direction G 
at which this ray enters his eye, and the impression conveyed is 
that the real tree is standing on the bank of a lake, in which its 
inverted reflection is seen. The cold surface of the sea may some- 
times so chill and render relatively dense the lower layers of the at- 
mosphere, that rays passing from 
a vessel at A (Fig. 42), completely 
hidden from the observer at C by 
the rotundity of the earth, are re- 
fracted downward by the rarer 
layers of the higher atmosphere, 
and totally reflected at B, thus 
producing an image of the con- 



-tas^D 




Fig. 42. 



cealed vessel in the clouds at D, above its true position, and in the 
direction which the ray entered the observer's eye. The image may 
sometimes be erect, sometimes inverted, and may be greatly en- 
larged. An image of a vessel 30 miles distant from the observer 
has thus been seen, and the image of the French coast, which is 
usually invisible, has thus been lifted into the view of those on the 
opposite side of the English Channel. Lateral displacement occurs 
when the layers of air of different density are vertical, or more or 
less inclined to the horizontal. 

Color of the Atmosphere. — In small masses, air has 
no appreciable color. In large masses its color varies 
with its position in relation to the sun. If the sunlight 
passes directly through the air to the eye, we see the air 
by transmitted light, and it is reddish if the sun is near the 
horizon, but yellowish if the sun is high in the heavens. 
If the air we observe is not directly between the eye and 
the sun, we see it by the sunlight which it reflects to the 
eye, and it appears azure, or bluish. It is the color of the 
atmosphere, thus seen, which makes the sky and distant 
hills or mountains appear blue. 

It has been seen (page 28), that a colorless ray of sunlight, 
passing through a prism, is refracted and separated into a num- 
ber of colored rays. The only difference in these colored rays 
is thought to be the width of the ether waves, or vibrations, which 
are supposed to constitute all light ; the blue is conceived to be 
caused by short, quick waves, and the red by longer and slower 



104 PHYSICAL GEOGRAPHY 

waves, while the original colorless ray is supposed to be composed 
of waves of all lengths, unassorted. Now, the atmosphere is known 
to contain myriads of floating dust motes and other particles, which 
vary in size from an inconceivable smallness to the size where they 
can no longer float, but fall through the air to the earth's surface. 
If an ordinary water wave encounter a small chip or other floating 
object, it simply passes under the object and continues its course. 
If, however, it encounters a much larger object, the wave is unable 
to lift it, and, striking against it, breaks and rebounds, that is, is re- 
flected from it. The floating dust motes have much the same effect 
on the light waves passing through the atmosphere. The shorter blue 
waves are broken and reflected not only by the larger motes, but by 
those which are too small to reflect the longer yellow and still longer 
red waves. Thus, when a ray of sunlight passes through the atmos- 
phere, more of its blue waves than of its yellow and red waves are 
reflected to the eye, and hence objects seen by such reflected sun- 
light, in which the short blue vibrations predominate, appear more 
or less bluish. When, however, a ray of sunlight passes directly 
through the atmosphere to the eye, more of its blue waves have been 
reflected into other directions, and the reddish and yellowish waves 
are in excess in the transmitted light. When the sun is near the 
horizon, its rays pass through great distances of the lower atmos- 
phere, which contains the largest motes, and the yellow as well as 
the blue waves are sifted out by this process of selective absorption 
of the atmosphere (page 28), leaving the transmitted light reddish. 
The remarkably long and brilliantly red sunsets and twilights of 
the fall and winter of 1883-4 are thought to have been caused by 
the ordinary action of this selective absorption. They were remark- 
able, however, because the amount of dust in the higher atmosphere 
was unusually large at that time, great quantities having been grad- 
ually diffused over the entire globe from the terrific volcanic erup- 
tion of August, 1883 — at Krakatoa, in the Strait of Sunda, between 
Sumatra and Java (page 285). 

Color of Clouds and Snow. — Masses of cloud and 
snow are nearly opaque, although composed of minute 
particles of transparent water or ice ; for while most of the 
light falling upon each particle is transmitted through it, 
and but little reflected from it, still there are so many par- 
ticles, and hence so many minute reflections, that all the 
light is reflected away from the eye before it can traverse 



LUMINOUS PHENOMENA. 



I05 



the mass of cloud or snow. When seen by directly re- 
flected sunlight, and not too distant, the sensation appro- 
priate to colorless opaque bodies is excited, and the cloud 
or snow appears milk white ; but clouds, if very distant 
and high above the horizon, appear bluish, while distant 
snow or clouds near the horizon appear yellow or red, be- 
cause the dense atmosphere reflects the blue waves of 
their rays away from the eye. 

The Rainbow is the beautiful arc, containing all the 
colors of the spectrum, which is usually seen through a 
shower or heavy mist, upon which the sun shines from a 
point behind the observer. It is caused by the separation 
of white sunlight into its prismatic colors by refraction in 
the water drops, and the total reflection of these colored 
rays back to the eye of the observer. Each color and 
each point in the arc is the instantaneous reflection from a 
separate drop. The exterior edge of the rainbow is red, 
and the interior edge is blue or violet. Sometimes a 
double arc or rainbow is seen, in which case the outer one 
is wider but fainter than the in- 
ner one, and the order of its 
colors is reversed. 

When a beam of sunlight enters a 
drop of clear water at a certain angle, 
it is totally reflected from the interior 
surface, and emerges on the same 
side of the drop at which it entered. 
In addition to this, while the beam is 
traversing the drop, refraction sepa- 
rates it into its colored rays, of which 
only the red, yellow, and blue ones 
are indicated in Fig. 43. If the position of this drop is such that the 
reflected red ray enters the eye of the observer, the other colored 
rays pass above the eye and the drop appears red. But at the same 
instant a similar phenomenon takes place in other drops at such 
distances below the first that only their yellow and blue rays re- 
spectively enter the eye, causing these drops to appear respectively 




Fig. 43- 




I06 PHYSICAL GEOGRAPHY. 

yellow and blue, — the lower drop appearing blue. Between these 
drops are others which reflect their appropriate color, and the whole 
series of drops gives the appearance of a continuous party-colored 
band, the red above gradually changing through yellow to blue 

below. Each drop occupies but for an 
instant the proper position for its re- 
flection to enter the eye, but this posi- 
tion is so soon occupied by a following 
drop that the sensation is continuous. 
The angle between the sunbeams and 
the reflected rays is always nearly 42 . 
Each part of the shower reveals pris- 
matic colors at a point where lines 
drawn to the sun and the eye inclose an angle of 42 . Collectively, 
these points form the curve of the rainbow. The second exterior 
rainbow sometimes seen is caused by the refraction and two reflec- 
tions of the sunbeam within the drop (Fig. 44). 

Halos and Coronas are sometimes seen around and 
at some distance from the sun or moon. The halos result 
from the refraction of light in ice crystals which compose 
the highest cirrus cloud, and are more or less distinctly 
colored. Coronas may be colorless, and are caused by the 
diffraction from the surface of haze- or cloud-globules. If 
these are small, the diameter of the corona is great, and 
vice versa; hence, when a ring is seen closely encircling 
the moon, the globules of water in the clouds are known 
to be large, and rain may be expected. 

Atmospheric Electricity. — The atmosphere is always 
more or less highly charged with electricity. This is prob- 
ably a result of evaporation, and the friction of air and 
vapor particles with each other. Millions of vapor parti- 
cles condense into a single cloud-globule ; hence, however 
small the electric charge on a single vapor particle, the 
accumulation in a cloud mass might be enormous. 

Lightning. — When an electrified cloud approaches 
another cloud or the earth sufficiently close, its electricity 
and the opposite kind induced on the neighboring cloud or 



LUMINOUS PHENOMENA. IOJ 

the earth, rush together through the intervening air, pro- 
ducing the great electric spark called lightning. This elec- 
tricity travels with the enormous velocity of radiant light 
and heat (186,000 miles a second). A flash of lightning, 
therefore, though often one mile, and sometimes more 
than five miles in length, seems instantaneous. There are 
at least three varieties of lightning: (1) forked lightning, 
(2) sheet or heat lightning, and (3) ball lightning. 

Forked lightning is a sharp, zigzag line of dazzling white light, 
marking the line of least resistance through the dense lower air be- 
tween two highly charged clouds, or a cloud and the earth. Sheet 
lightning is the most common form, and occurs as a broad sheet of 
rather pale, diffused light. Frequently it is not accompanied by 
audible thunder. Usually it is distant forked lightning, but some- 
times is a weak electrical discharge within a cloud at a considerable 
height where the air is rare. Ball lightning is a very rare form. A 
vivid flash, accompanied by a violent explosion, seems to project a 
brilliant bomb to the earth. Upon striking the earth, the bomb may 
rebound several times before it splits up and disappears. No satis- 
factory explanation has been given of this singular form of electrical 
discharge. 

Thunder is simply the crackle of the lightning spark. 
It has the same cause (page 34) as the much feebler 
crackle of the smaller sparks produced artificially. The 
passage of electricity is so rapid that the crackle is pro- 
duced at practically the same instant throughout the 
length of a lightning flash several miles long. But as 
sound requires almost five seconds to travel one mile, it 
arrives from successively more distant points at sensibly 
later periods of time, and thus produces the continuous 
roar or roll of thunder. The sound is further prolonged 
and repeated by being reflected, or echoed, from the sur- 
faces of clouds, the earth, and masses of air of unequal 
density. Thunder is seldom heard over a greater distance 
than twelve miles. 



108 PHYSICAL GEOGRAPHY. 

St. Elmo's Fire. — Atmospheric electricity of very low 
intensity, such as often occurs in fair weather, is frequently 
sufficient to induce in prominent, sharp-pointed objects, a 
greater amount of electricity than the attenuated object 
can hold, and what is called a brush discharge takes place, 
without audible noise, but frequently with a feebly lu- 
minous glow. This glow is often seen at the ends of 
lightning rods or of the masts and spars of vessels, and is 
called by the sailors St. Elmo's Fire. 

The Aurora Polaris, or Polar Light, is a singular and 
beautiful phenomenon seen in the sky, most frequently in 
high latitudes, but occasionally in all parts of the earth. 
It consists of luminous clouds, arches, or rays. The rays 
frequently shoot up and down in diverging lines from the 
horizon, and appear to converge in the zenith. The 
aurora is usually a pale, greenish yellow, but sometimes 
is crimson, violet, or steel blue. In the northern hemi- 
sphere, the phenomenon is most common in a narrow 
zone surrounding, but at some distance from, the magnetic 
pole of the earth. 

This zone embraces the Faroe Islands, and crosses central Labra- 
dor, Hudson Bay, and Point Barrow in northern Alaska, and then 
skirts the north coast of Asia. North of this zone the aurora is gen- 
erally seen in the southern sky, but south of the zone in the northern 
sky. The height of the aurora varies greatly, but its average alti- 
tude seems to be about ioo miles, and hence in a region where the 
atmosphere is exceedingly rare. The cause of the aurora is un- 
known ; it is certainly connected with the magnetism of the earth, 
and probably results from the discharge of atmospheric electricity. 



PART III.— THE SEA. 



CHAPTER VIII. 

DEPTH, COMPOSITION, AND TEMPERATURE. 

Thy way is in the sea, and thy path in the great waters. — Psalm lxxvii : 19. 

The Sea is a continuous body of water which partly 
envelopes the earth, forming nearly three fourths (73)4%) 
of its surface. 

Oceans. — The polar circles, the continents, and the 
meridians from their southern points are taken as the 
boundaries of five great divisions of the sea, called oceans, 
which vary greatly in shape and extent. 

The Pacific is the largest ocean. It is oval in shape. 
The greatest width in an east and west direction lies along 
the equator, and is about 10,000 miles. Its length, from 
Bering Strait to the Antarctic Circle, is 9,000 miles. It 
embraces 71,000,000 square miles, — about one half (49%) 
of the total sea area. 

The Atlantic Ocean is next in size. It is a long and 
narrow channel, extending 9,000 miles between the polar 
circles, with an average width of 3,600 miles. Its area is 
34,000,000 square miles, or 24% of the sea surface. 

The Indian Ocean is roughly circular in shape, having 
a diameter of about 6,300 miles, and an area of 28,000,000 
square miles, or 20% of the sea surface. 

P. G.— 7. ( I09 ) 



IIO PHYSICAL GEOGRAPHY. 

The Antarctic Region, lying within the Antarctic Circle, 
is circular, with a diameter of 3,300 miles, and an area of 
7,000,000 square miles, or 4^^ of the sea surface. 
About 4,700,000 square miles of this region have never 
been explored. The unexplored region is supposed to 
contain a low continent or large island-group completely 
covered by a continuous ice cap more than 2,000 feet 
thick, which terminates on all sides in a perpendicular 
cliff of ice about 200 feet high above sea-level. 

The Arctic Ocean is really a great gulf of the Atlantic, 
extending for 3,300 miles from Iceland to Bering Strait. 
It has a width of less than 2,500 miles, and an area of 
4,000,000 square miles, or 2^% of the sea surface. 

the r,ioBE 1 — 1 

the sea TiiTntraviiiiBiifTwmiMiii~fiwiBiirwMrimi 1~ ■ 

PACIFIC HHBBHBHBBa^raM 

ATLANTIC S^nBHBl 

INDIAN wow «■ 

ANTARCTIC REGIONS— BO 

ARCTIC ill 

UNITED STATES □ 

Fig. 45.— Relative Areas of the Oceans. 

Continuity of the Sea. — The Pacific, Atlantic, and 
Indian oceans are wide and open at the south, where, 
together with the Antarctic, they form a continuous and 
uninterrupted sea as far north as Cape Horn, a point cor- 
responding in latitude to central Labrador and Denmark 
in the northern hemisphere. From this sea each ocean ex- 
tends northward as a great bay or channel. At the tropic 
of Cancer the Indian Ocean encounters Asia and ends; at 
the Arctic Circle the Pacific practically ends at the shallow 
and narrow Bering Strait, leaving the Atlantic alone to 
make a broad connection with the Arctic Ocean, and carry 
the continuity of the sea into the frozen regions about the 
north pole. 



DEPTH OF THE SEA. Ill 

Atlantic Coast. — The Atlantic, and its northern ex- 
tension, the Arctic Ocean, send the greatest number of 
deep indentations into the land. In general these inden- 
tations have comparatively narrow mouths, and form great 
inland seas. Thus, to the Atlantic and Arctic basin be- 
long the Gulf of Mexico, Hudson Bay, the Gulf of Obi, 
White, Baltic, Mediterranean, and Black seas. For this 
reason the Atlantic, although it has but one half the area, 
has a longer coast-line than the Pacific. The Atlantic 
coast is 55,000 miles long; that of the Pacific but 47,000. 

The Pacific Coast, while much more regular than that 
of the Atlantic, possesses the greatest number of border 
seas, partially separated from the main ocean by chains of 
islands. Such are Bering, Okhotsk, Japan, Yellow, and 
China seas, the seas of the Malay Archipelago, the Coral 
Sea east of Australia, and the expanses of water within 
the numerous islands of southern Chile, British America, 
and Alaska. 

The Indian Ocean is peculiar in the number and size 
of the great open-mouthed indentations in its coast-line, 
such as the Gulf of Aden, the Arabian Sea, the Bay of 
Bengal, Timor Sea, and the Great Australian Bight. 

Average Depth. — The average depth of the sea is 
2,150 fathoms (1 fathom equals 6 feet) or 2~y miles. 
That of the Pacific is 2,500 fathoms or about 3 miles; 
that of the Atlantic and Indian oceans is 2,000 fathoms 
or 2 y miles ; and that of the polar oceans is probably 
less than 1,000 fathoms or about 1 mile. While these are 
the average depths, there are places where the depth is 
much greater, and others where it is much less. The blue 
shading in the charts indicates the portions of the earth's 
surface that would still be covered with water were the 
surface of the sea lowered 2,000 fathoms. The white por- 
tion of the chart indicates the region that would thus be 



112 



PHYSICAL GEOGRAPHY. 



DEPTHS OF THE SEA 

PACIFIC 
OCEAN 




converted into dry land. A depression of the sea surface 
of 4,500 fathoms or about 5 miles, would be required to 
convert the whole surface of the earth into dry land. 

The dotted line in the white portion of each chart indicates the 
shore-line of the sea were its surface lowered only 1,000 fathoms; 
the darker blue tinting indicates regions that would still be covered 
with water were the present sea surface lowered 3,000 fathoms — t>% 
miles — while the small areas of solid blue east of Japan and the West 
Indies indicate the deepest depressions of the earth which would 
remain sea were the present surface lowered 4,000 fathoms or \]/ z 
miles. 

Depths of the Sea Compared with Heights of the 
Land. — The greatest depressions of the earth are about 



DEPTH OF THE SEA. 



113 



Relative extent of high 
land.— 

H Land high 
^■tnan 6000 
^SLowerland^ 



DEPTHS OF the SEA 
ATLANTIC and 
INDIAN 

OCEANS. 




as far below, as the highest mountains are above the sea 
surface; but the area of deep depressions is very much 
greater than that of high elevations. Thus, about 83% of 
the sea area, or 114 million square miles, is more than 
1,000 fathoms deep; while but 9% of the land, or about 
5^ million square miles, has an elevation greater than 
1,000 fathoms (6,000 feet) above sea-level. This elevated 
region of the land is indicated on the chart by the darkest 
red tint. It is estimated that it would require all the 
solid portion of the planet down to a depth of 1,600 
fathoms below sea-level to fill the greater depressions up 
to that depth. 



114 PHYSICAL GEOGRAPHY. 

Configuration of the Sea Bottom. — The sea bottom 
is much smoother than the surface of the land. It sinks 
more or less rapidly from the shores of the continents to 
its average depth, and continues as a vast, gently undulat- 
ing plain to the opposite continent. 

Submarine Plateaus. — Occasionally an undulation 
may rise gradually to an elevation a mile or two above the 
plain, and continue for a greater or less distance as a 
plateau, at a depth of 1,000 or 2,000 fathoms, before 
again sinking. The narrower of these plateaus or ridges 
correspond in a general way with the broad plateaus of the 
land, but it is probable that the sea bottom far from land, 
contains no narrow and rugged irregularities comparable 
with mountain chains. 

The Atlantic. — A submarine plateau, or broad ridge, extends 
along the middle of the Atlantic throughout its length, and is con- 
nected with the American continent by a spur near the equator. 
The average depth on this plateau is 1,500 fathoms, while the three 
depressions into which it divides the basin of the Atlantic sink to 
mean depths of over 2,500 fathoms. 

The Pacific basin is divided by a submarine ridge, which also ex- 
tends along its greatest length, or from northern Chile to Japan. 
Along its crest the average depth is about 1,500 fathoms. The de- 
pression to the north sinks to a general depth of nearly 3,000 fath- 
oms, and to much greater depths in the west. To the south of the 
ridge, the floor of the Pacific is separated by spurs from the main 
ridge into several distinct depressions, each of which reaches a depth 
of nearly 3,000 fathoms. 

The Indian Ocean seems to be freer from submarine plateaus than 
either of the other oceans, but a short one appears east of Madagas- 
car, and possibly another extends south from the west coast of India. 

Composition of Sea-water. — Unlike rain-water, or 
that which is common in lakes and rivers, the water of the 
sea is so salt and so bitter as to be undrinkable. If 100 
pounds of sea-water be placed in a clean vessel and 
allowed to evaporate, about y/ 2 pounds of solid matter 



COMPOSITION OF SEA- WATER. 115 

will remain after the liquid has disappeared. This solid 
matter, dissolved in sea-water, makes it heavier or denser 
than fresh water, and gives to it the peculiar taste. 

The amount of solid matter, and hence the weight or density of 
the surface water, varies slightly in different parts of the open sea, 
being greatest in the trade wind regions, where evaporation is 
greater than the rain-fall, and least in equatorial regions where the 
rain-fall is in excess, and in the polar seas, where the melting ice 
supplies a great amount of fresh water. In partially inclosed seas 
or bays the amount of solid matter in solution may increase to about 
4$, as in the Mediterranean and Red seas ; or decrease to 2^$,, as 
in New York Bay, or to 1%%, as in the Black and Baltic seas, ac- 
cording as the evaporation and the salt water received from the 
ocean is greater or less than the amount of fresh water received 
from rain-fall and rivers. The relative density or saltness of differ- 
ent parts of the sea is shown in the chart on page 138. 

The Solid Matter. — While the amount of solid matter 
varies slightly, its composition remains practically the same 
in all parts of the sea. Rather more than three fourths of 
it is chloride of sodium, or common salt. The sea thus 
contains in solution enough common salt to form a solid 
layer 126 feet thick over the entire globe. The remaining 
portion of the solid matter (other than salt) gives the pe- 
culiar bitter taste to sea-water, and consists of chloride of 
magnesia, Epsom salts, gypsum, and traces of nearly 
every known mineral, minute quantities of each being 
dissolved in water percolating through the rocks of the 
land, and carried eventually by the rivers to the sea. 

The percentage of the principal solids dissolved in sea-water is 
as follows : 

Chloride of sodium (common salt) 77758% 

Chloride of magnesia 10.878 

Sulphate of magnesia (Epsom salts) 4-737 

Sulphate of lime (gypsum) 3.600 

Sulphate of potassium 2.465 

Carbonate o-f lime (limestone), and all others . 0.562 



Il6 PHYSICAL GEOGRAPHY. 

Gaseous Matter. — In addition to its solid or mineral 
ingredients, sea-water always contains, dissolved, a greater 
or less quantity of the atmospheric gases, — oxygen, nitro- 
gen, and carbonic acid. Bubbles of air composed of these 
gases become entangled in the waves of the sea surface, 
and the gases are dissolved and gradually diffused to 
the greatest depths. The quantity of gas so dissolved 
amounts to from 2 f to 3 % of the volume of the sea (equal 
to a layer of air 230 feet thick surrounding the earth), in 
the proportion of about ^ oxygen, ^ carbonic acid, and 
^ nitrogen. The oxygen is a little more abundant in the 
water near the surface, and the proportion of carbonic acid 
increases toward the bottom. It is the oxygen thus dis- 
solved in sea-water which enables submarine animals to 
live. They inhale it and exhale carbonic acid. 

Cause of the Saltness of the Sea. — It is believed 
that the mineral ingredients of sea-water were principally 
derived from the mineral gases in the atmosphere, when 
its water vapor first condensed to form the sea, at an early 
period of the planet's history ; and hence that sea-water 
has always been salty. 

At that time, the earth's surface was much hotter than it is now, 
and great quantities of the minerals which exist as solids at the 
present temperatures, existed then as gaseous components of the at- 
mosphere. Under the enormous pressure of such an atmosphere, 
vapor might condense into clouds and rain at temperatures now re- 
quired to melt iron. Hot water dissolves a much greater quantity 
of most minerals than cold, and such hot rain falling through such a 
mineral laden atmosphere would reach the earth strongly impreg- 
nated by the mineral gases through which it had passed. 

There are processes now at work, however, which, 
by continually adding small quantities of similar minerals, 
tend to gradually increase the saltness of the sea. Sea- 
water, in evaporating, leaves all its impurities behind. 
The vapor condenses and falls as nearly pure rain-water. 



TEMPERATURE OF THE SEA. 117 

Part of it falls on the land, and only reaches the sea again 
after a long journey in some stream or river. During this 
journey it dissolves and carries away, in solution, minute 
quantities of the soils and rocks with which it comes in 
contact. River water, however clear, is thus never pure. 
Upon entering the sea, it adds its mite to the quantity of 
mineral matter in solution. 

While there are 350 parts of mineral matter in 10,000 parts of 
sea-water, there are but 2 parts of mineral matter in an equal quan- 
tity of river water ; and being in so small a proportion, it does not 
appreciably affect the taste. The solution in river water depends 
upon the nature of the rocks encountered. The minerals of lime- 
stone, granite, and sandstone, which constitute so large a portion 
of the rocks of the earth, form about three fourths, and common 
salt but a small part of the mineral matter in river water, while 
the proportions of the substances are just reversed in sea-water. 
The reason for this is explained on page 243. 

Temperature. — In general, the surface water of the 
sea is the warmest. Its temperature varies from about 
8o° near the equator to about 30 near the poles. The 
temperature of the water on the sea bottom, however, is 
about 35 under the equator, and about 29 under the 
polar oceans. Thus, while there is a difference of 50 be- 
tween the surface temperatures of the polar and equa- 
torial oceans, there is a difference of only 6° between the 
temperatures of the water on their bottoms. 

Surface Waters. — The great difference between the 
temperatures of polar and equatorial surface waters, results 
from the different heating power of the sun's rays when 
falling almost vertically, as near the equator, and very 
obliquely, as near the poles. As the sun is vertical over 
the tropic of Cancer in June, and over the tropic of 
Capricorn in December, the surface waters in either hemi- 
sphere are alternately warmer and colder, according to 
the season of the year. This seasonal difference of tern- 



I I 8 PHYSICAL GEOGRAPHY. 

perature is very slight near the equator and near the poles, 
but in the oceans of the temperate zones it amounts to 
about io°. Thus, in the latitude of New York, the tem- 
perature of the sea surface is between 50 and 6o° in 
winter, and between 6o° and 70 in summer. 

Water Beneath the Surface. — Since water is a very 
poor conductor of heat, the direct influence of the solar 
rays is confined to a comparatively thin layer of surface 
water. From the surface the temperature first falls rapidly 
as the depth increases, then more slowly, and then with 
extreme slowness, either to the bottom or to a certain 
depth, whence it remains nearly uniform to the bottom. 



ATLANTIC OCEAN 

s St. Pauls Rock 



PACIF.IC OCEAN 

Sea . ffleutian la. /5 Hawaiian Is. Samoa Is. * ... Friendly Is. 

SURfACE l ''""-T^y ^jP 



2000 FA 
4000 FA. 



Fig. 46, 

Warm and Cold Water of the Sea. — Even under 
the equator, a temperature of 40 is always reached within 
a depth of 800 fathoms, and in higher latitudes at less 
depths. Thus, the great mass of sea-water has a temper- 
ature below 40 , or removed but a few degrees from its 
freezing point. The portion of the sea which has a tem- 
perature above 40 forms a comparatively thin layer at the 
surface of the temperate and equatorial oceans. In the 
sectional diagrams, this layer is indicated by solid black. 

Ice of the Sea. — Salt water freezes at a lower temper- 
ature than fresh on account of its saltness. The surface 
water of the sea does not begin to solidify into ice until 
its temperature falls below 29 Fahrenheit. 



ICE OF THE SEA. 119 

In freezing, salt water discards most of its salt, and expanding, 
becomes comparatively fresh ice. The discarded salt, mixing with 
the water immediately beneath the ice, makes it Salter, and there- 
fore it does not freeze, although as cold as the surface. It also 
makes it heavier, and causes it to sink and cool the deeper water, 
which is thus eventually cooled to about 29 entirely to the bottom. 

Ice-fields and Floes. — In the frigid zones, the sur- 
face of the sea is annually frozen into vast fields of thick 
ice, hundreds of miles in extent. The movements of the 
water and variations in its temperature cause the ice to 
crack into immense pieces, or floes, the pressure of which, 
one against the other, breaks off and squeezes up huge 
fragments, until the whole surface of the floe becomes so 
rough and uneven that traveling over it is almost impossi- 
ble. The portions of the ice-field near the shore are fre- 
quently covered with masses of rock and soil loosened 
from overhanging cliffs by the frosts of the long Arctic 
winter. Thousands of tons of such land rubbish are thus 
annually carried to sea when the ice-floe becomes de- 
tached in the early summer, and may be transported hun- 
dreds of miles before the ice melts and allows its load to 
sink to the bottom. 

Icebergs. — Very different from the comparatively low 
ice-floe, both in appearance and in manner of formation, 
are the great icebergs, sometimes 200 or 300 feet high, 
occasionally seen floating in the Atlantic as far south as 
the latitude of Washington, and frequently observed 
stranded in the shallow waters around Newfoundland. Un- 
like the ice-floe, bergs are not frozen sea-water, but are 
land ice. They are formed of the snow, which, falling to 
great depths on polar lands, and accumulating in the con- 
tinued cold to still greater depths, becomes consolidated 
under the pressure of its own weight into solid ice, which 
covers the greater part of the land, and increases in thick- 
ness with the accumulation on its surface, until its weight 







(120) 



ICE OF THE SEA. 121 

causes it to move gradually downward as a glacier over the 
surface of the land into the sea. When it has advanced to 
a depth of water greater than about nine tenths of the 
thickness of the ice sheet, its buoyancy causes great 
blocks to break off and drift away as icebergs. 

Observations at the foot of the Muir glacier lead Professor G. Fred- 
erick Wright to believe that most icebergs owe their detachment 
from the parent glacier, not to the buoyancy of the ice, but to the 
fact that the advance of the glacier is faster near its surface than 
near its bed (p. 233). Fragments of the surface ice, of greater or 
less volume, are thus pushed off over the submerged foot of the gla- 
cier, and these fragments float away as icebergs. 

In Antarctic seas, the icebergs are usually about 175 feet high, and 
sometimes 3 miles long, with flat and nearly level tops. As only 
about one tenth of the mass of a berg protrudes above the heavy 
sea-water, such icebergs must extend to a depth of about 1,750 feet. 
As the iceberg drifts with the currents into warmer latitudes, it be- 
comes very irregular in shape through unequal melting, and thus 
may turn completely over several times, and reach comparatively 
low latitudes before its great mass is entirely dissolved. In its jour- 
ney over the land and sea bottom before it breaks away from the 
parent ice sheet, great masses of stone and gravel, becoming em- 
bedded in its under surface, are torn from their places and distrib- 
uted over hundreds of miles of sea bottom by the gradual melting 
of the iceberg. The chart on page 138 shows the regions in which 
floating ice may be encountered. 



CHAPTER IX. 



WAVES AND TIDES. 



Thou rulest the raging of the sea; when the waves thereof arise, thou stillest 
them. — Psalm lxxxix : 9. 

Movements of the Sea. — The water of the sea is in 

constant motion. Its movements may be broadly divided 
into three different classes, namely, waves, tides, and cur- 
rents. 

Waves. — The ordinary waves of the sea are caused di- 
rectly by the impact or friction of the wind. A small 
local agitation of the surface water is thus produced, which 
spreads rapidly in all directions as a succession of undula- 
tions or waves. This phenomenon may be produced by 
blowing upon or along the surface of the water in a basin. 
Under the continued action of the wind, these waves soon 
grow, in the deep and open sea, into great billows. Be- 
yond the region of the wind which caused them, the bil- 
lows advance with gradually diminishing height, and may 
even reach the shores of the continents. As there is 
never a time when the winds are not blowing and creating 
waves in some parts of the sea, its surface, even in regions 
where the wind is not blowing, is almost always heaving 
with the " groundswell, " or the diminishing undulations 
of waves created in some other regions. 

Movement of the "Water in Waves. — While the 

undulation advances rapidly in one direction, the water 

itself does not partake of this continuous progressive 

movement. The motion of the water is indicated by that 
(122) 



WAVES AND TIDES. 



123 



of a floating cork, which is observed to rise and fall as the 
wave passes, but otherwise to remain in nearly the same 
position. 

In reality, the water advances while on the upper half and re- 
cedes while in the lower half of the wave, each particle moving in a 
nearly circular path whose diameter equals the height of the passing 
wave. This is illustrated in Fig. 47. A cork at a, on the front slope 
of an advancing wave, A, reaches b as the wave reaches X, c when 
the wave arrives at V, d as the wave reaches Z, and a again when 
the wave has advanced its length to B. As the cork advances when 
above and recedes when below the medial line ef, and as the por- 
tion of the wave ae above that line is shorter than the portion af 
below, the advance of the cork, while equal in amount, is more 



Direction of motion of u/.avg sss- 




Fig. 47- 

rapid than its recession. The enormous energy of waves is due to 
this slight but exceedingly rapid (page 17) advance of the great 
body of water while in the crest of the wave. 

Size and Speed of Waves. — The size of waves 
(height from trough to crest, and length from crest to crest) 
in the open ocean depends upon the force and continuance 
of the wind. The speed increases with the size. The 
largest wind waves are about 50 feet high and half a mile 
long, and travel at the rate of 80 miles an hour. The 
ordinary storm waves are not more than 30 feet high and 
600 feet long. They travel about 37 miles an hour. 

Depth of Water Affected by Waves. — In the waves 
of the open sea, all motion is confined to a comparatively 
thin layer of surface water. At a depth equal to the 
wave length, the motion is less than 5-g-o-th part of that at 
the surface. Thus, the largest waves of the sea, which 



124 PHYSICAL GEOGRAPHY. 

attain a height of 50 feet, cause a corresponding move- 
ment of but one inch at a depth of half a mile (440 
fathoms), while the motion of smaller waves becomes in- 
sensible long before a depth equal to their length is 
reached. The motion arising from the ordinary waves of 
the sea is probably quite insensible at a greater depth than 
100 feet (17 fathoms). 

Breakers. — When waves from the deep open sea reach 
water so shallow that perceptible wave motion reaches to 
the bottom, the increased friction retards the bottom of 
the advancing wave, and those following begin gradually 
to overtake it. The waves thus become shorter and 
higher, and their slopes, especially the front slope, be- 
come steeper as the water becomes shallower. Finally, 
the front slope of the wave in the shallowest water be- 
comes so steep that the water in its crest falls forward in 
a graceful curve, and dashes upon the beach, amidst foam 
and spray, as a breaker. 

Small waves, caused directly by the wind, break thus where the 
depth of water below the trough has decreased to about one half 
the height of the wave. The larger undulations of the ground- 
swell, however, sometimes break when the depth below the trough 
is twice the height of the wave. Waves may thus break far from 
the shore if the water is sufficiently shallow. "White-caps," seen 
on the crests of waves in deep water when a brisk wind is blowing, 
are simply the curling over and lashing into foam of the surface 
water by the wind, when exposed to its full force at the highest part 
of the wave. 

The Force of Waves. — The force with which waves 
break against the shore depends upon their size, which in 
turn depends upon the force and direction of the wind and 
the extent of its contact with the water surface. Thus, 
the waves are stronger when the wind blows toward a 
coast than when it blows seaward. They are stronger 
upon a coast facing the open sea, than upon one protected 



WAVES AND TIDES. I 25 

by outlying capes or islands. As the prevailing winds in 
the temperate zones are toward the east, it is generally 
true that in those zones the eastern shores of the oceans 
have the heavier waves ; while in the torrid zone, for a 
similar reason, the heavier waves are on the western 
shores. The average force of breakers on the eastern 
shores of the temperate oceans is about 600 pounds to 
the square foot in summer, and about 2,000 pounds in 
winter, when the winds are stronger. 

The force of the waves at the eastern end of Lake Erie has been 
sufficient to tear from its bed in the masonry at Buffalo harbor a 
rock weighing half a ton, and after moving it several feet, to turn 
it upside down. In the Shetland Islands, in the eastern part of the 
Atlantic, storm waves have torn blocks of stone weighing from 5 to 
19 tons from their natural beds 70 feet above sea-level, and carried 
them many feet inland ; while at Wick, on the north-east coast of 
Scotland, a mass of masonry weighing 1,350 tons was removed en- 
tire from the end of the breakwater by repeated blows of storm 
waves in December, 1872, and a mass weighing 2,600 tons was simi- 
larly removed in 1877. 

Tides. — An observer on the shore soon recognizes the 
existence of other wave-like movements of the sea, quite 
different from that of wind waves. These movements are 
called the tides. Tidal waves differ from wind waves in 
being more regular, and in being much longer in propor- 
tion to their height. They are so long that, although 
they travel much faster than wind waves, it takes about 
twelve hours for one to travel its length. They are so 
flat that, upon the open coast, they never form breakers 
as the wind waves do. 

The approach of the crest of a tidal wave is indicated on the 
shore by the gradual rise of the sea surface, and the flooding of 
great areas of low coast ; hence, the front slope of the tidal wave is 
called flood tide. When the crest arrives at the shore, the sea sur- 
face, having nearly reached the top of wharves and piers, stops ris- 
ing. It is then high tide. In a short time the sea surface gradually 
sinks, and the water slowly flows off or ebbs away from the sub- 



126 



PHYSICAL GEOGRAPHY. 




Fig. 48.— High and Low Tides. 



merged flats, giving rise to the name ebb tide for the front slope of 
the trough of the tidal wave % The arrival of the trough marks low 
tide, for the sea surface stops sinking and soon begins to rise on 
the front slope of the following crest, and the phenomena are re- 
peated. 

Tidal Currents. — Tidal waves differ, also, from wind 
waves in the movement of the water. Wind waves cause 
no current by which a floating cork is carried any great 
distance during the passage of a wave. Tidal waves, on 
the contrary, are created by strong currents, in which 
water is carried forward long distances on the crest of 
the wave, and backward long distances in its trough. 



WAVES AND TIDES. 



127 



The movement of a floating cork during the passage of 
a tidal wave describes a greatly elongated oval ; thus, sup- 
pose A, B, C, etc., (Fig. 49), to be equally distant points in 
a tidal wave advancing to the right. Suppose a cork to be 
floating at A. The points b, c, d, etc., indicate the posi- 
tion of the cork when the corresponding points of the 
wave B, C, D, etc., respectively pass under it. It will be 
noticed: (1) That when above half tide level the cork 
moves forward ; when below that level, it moves backward. 
(2) That at half tide level (A and f) there is the most 
rapid rise or fall, but little or no current ; hence, this stage 
of the tide is called slack water. (3) That near high tide 
(c , d) and low tide {h, i) there is little rise or fall, but the 
current is swiftest. 

Depths of Water Disturbed by the Tide. — 

While wind waves disturb only a thin layer of 
the surface water, the tidal waves are caused by 
movement in the water clear to the bottom of 
the deepest sea. The lesser movement, or the 
rise and fall of the tide, decreases from the sur- 
face to the bottom; but the greater forward and 
back movements, or the tidal currents, exist at 
all depths. The current is slower near the bot- 
tom on account of the increased friction. 

Length and Velocity of Tidal Waves. — 
The length of a tidal wave, and the speed at 
which it travels, are greater in deep than in shal- 
low water. Its speed is such that, except in very 
shallow water, the wave travels its length in 12 
hours and 26 minutes, or about half a day ; hence, 
tides are semi-diurnal. Really, each wave re- 
quires 26 minutes more than half a day to travel 
its length ; hence, each second wave arrives at the 
shore about 52 minutes later than the wave of 
the day before. 

In water three miles deep, the tidal wave is nearly 
6^000 miles long, and travels nearly 500 miles an hour. In 



128 



PHYSICAL GEOGRAPHY. 



water 40 feet deep the wave is but 300 miles long, and travels but 
25 miles an hour. At high and low tide the speed of the tidal cur- 
rents is as great as that of the tidal wave, and this accounts for the 
energy which the tidal currents display in piling up bars or scouring 
out channels about the mouth of harbors. At other stages of the 
tide, however, the tidal currents are much slower than the tidal 
wave. In Fig. 49 it may be noticed that the cork travels only from 
A to f while the wave is traveling from F to f. But in the diagram 
the length of Af is greatly exaggerated ; in the deep open sea, Af 
is only about 600 feet, while Ff is about 3,000 miles ; hence, in 
such a locality the average speed of the tidal current is but 100 feet 
an hour, though the wave travels 500 miles an hour. 

Height of Tides. — In the deep open sea, the rise and 
fall of the tide is quite insensible ; it is probably less than 
two feet. When the tidal wave strikes the coast, how- 
ever, its slight rise and fall 
becomes perceptible by com- 
parison with the immovable 
land. The first land encount- 
ered by the tidal wave advancing 
from seaward are the ends of the 




capes which project farthest into the 
sea. At these points the rise and fall 
of the tide is generally less than at any 
other point of the coast, for as the cur- 
rents on the crest of the tidal wave carry the 
Fig. 5°- water forward into the bay between two ad- 

jacent capes, the water can find room for itself, as the bay 
becomes narrower and shallower, only by rising higher. 

Thus, the tidal wave of the Atlantic (Fig. 50) reaches capes 
Florida and Hatteras, and Nantucket Island at about the same time. 
At each of these points the rise and fall of the tide is between one 
and two feet, while at Savannah and at Cape May, near the heads 
of the intervening bays or bights, the height of the tide is 7 and 5 
feet respectively. This effect is still more marked in the Bay of 
Fundy, where, entering with a height of 8 or 9 feet, the tidal wave 
gradually increases to a height of over 40 feet at the head of the bay. 



WAVES AND TIDES. I 29 

Duration of Flood and Ebb Tides. — Upon open 
coasts the front and rear slopes of the tidal wave are 
nearly equally steep, and the trough is about half-way 
between the two crests ; hence, the flood and ebb tides 
are of equal duration — each about six hours. In com- 
paratively shallow and gradually narrowing bays, the slopes 
become steeper, for the wave length is less and its height 
greater; and, as the water when in the crest of the wave, 
being farther from the bottom and less retarded by fric- 
tion, moves forward faster than it moves backward when in 




the trough, the wave gradually changes its shape as it ad- 
vances, as indicated in Fig. 5 1 . Therefore, in bays and 
estuaries the flood tide is generally of shorter duration 
than the ebb tide. Thus, at the mouth of Delaware Bay 
the flood and ebb tide each continues for about six hours ; 
at Newcastle, Delaware, the tide rises for 5*^ hours and 
falls for about 7 hours, while at Philadelphia the flood 
lasts less than 5 hours and the ebb more than y 1 /^. 

The shape, depth, and situation of some estuaries is such that the 
front slope of the tidal wave becomes so steep that the crest falls 
forward into the trough, giving the flood tide the form of a breaker 
called the bore, which advances up the estuary at the great speed of 
the wave. In this case the flood tide is but momentary, while the 
ebb lasts about twelve hours. In the deep channel of the estuary, 
the tide may advance as a steep wave, and along its shallower mar- 
gins as a breaker or bore. The bore is seen at the head of the Bay 
of Fundy, in the Hoogly mouth of the Ganges, in the Dordogne in 
France, the Severn in England, the Amazon in Brazil, etc. 

Races. — Since the height of the tidal wave depends so 
largely upon the shape of the adjacent shores and the 



P. G.-8 



I3O PHYSICAL GEOGRAPHY. 

depth of the water, the same wave may rise to very- 
different heights in neighboring bays on the same coast. 
If the heads of these bays are connected by a narrow 
channel, the difference in water level in the two bays will 
give rise to a race, or strong current, in the channel, flow- 
ing at high tide, out of the bay in which the tide is high- 
est. But at low tide the water is lowest in this bay; 
hence, the direction of the race is reversed with each 
change of tide. 

Such races are common on all irregular coasts, especially when 
fringed with islands. Such are the famous "Maelstrom" among 
the Lofoden Islands, the currents of Pentland Firth north of Scot- 
land, and those of Hellgate, in the narrow channel between Long 
Island Sound and New York Bay. If the waters of the Sound 
could be separated from those of the Bay by a partition at this 
point, the water at high tide on the Sound side would stand 5 feet 
higher, and at low tide 5 feet lower than the water on the Bay side. 

Cause of the Tides. — The great regularity in the re- 
currence of the tidal wave denotes that it must be caused 
by some constant and regular force; and the fact that the 
water to its greatest depths is disturbed by it indicates that 
this force can not be a merely superficial one, like the 
wind. A force, constant, regular, and not superficial is 
found in the mutual attraction of gravitation between the 
earth, the moon, and the sun, and it is the peculiar effect 
of this force upon the liquid sea which results in the tidal 
wave. As the effect of the moon is usually most promi- 
nent, it is common to speak of the tides as caused solely 
by the moon. They are, however, always modified to a 
greater or less extent by the sun and the earth. 

Let A B D C (Fig. 52) represent the earth and let il/represent the 
moon. The gravitation of the moon attracts every particle of the 
earth with a force which varies inversely with the square of the dis- 
tance between the particle and the moon, the average attraction being 
that exerted on the particle at the earth's center, E. It is obvious 
that the moon's attraction on the side of the earth which is turned 




WAVES AND TIDES. I3I 

towards and is hence nearer to the moon is slightly greater than the 
average, while the attraction on the more remote side of the earth is 
slightly less than the average. There is consequently a tendency for the 
particles on the side of the earth towards the moon to move towards 
that luminary, and for the particles on the remote side of the earth to 
move away from the moon. The solid land resists this tendency to move, 
but the liquid sea yields to it, and slow movements throughout its depth 
set in from all directions toward C and B, by which the sea surface is 
slightly raised to H and F at points on opposite sides of the earth 
and correspondingly lowered to / and K along the meridian AED, 
half-way between //'and F. 
Owing to the earth's rota- 
tion, the points on its sur- 
face, towards which the 
tidal currents are moving, 
are constantly changing, 

and after about six hours, A lg ' 52 ' 

and D occupy the positions of C and B, I and K being then ele- 
vated, and H and F depressed ; that is, the currents in the sea 
which before moved away from A and D are reversed, and now 
move toward these points. About six hours later, when C occupies 
the position of B, the currents are again reversed, and so on. This 
regular reversal of the slow currents of the sea, which for about 
six hours advance from all directions toward a point, and then for 
a like time retreat in both directions away from this point, produces 
the long, low tidal wave, whose period of passage is always about 
twelve hours, — six hours for the crest and six hours for the trough. 
Tides occur later each day because, while the earth is making a 
rotation, the moon is advancing in the same direction in her orbit ; 
hence, the earth has to make a little more than a complete rota- 
tion to present the same point of its surface directly to the moon. 
For precisely similar reasons the sun also produces a tidal wave, 
but it is less than one half as high as that produced by the moon ; 
for, although the sun's attraction is much greater than the moon's, 
it is so much farther away that the diameter of the earth is rela- 
tively insignificant, and his attraction on opposite sides of the earth 
is nearly the same. Once a week, however, the existence of the 
solar tide becomes apparent. The moon makes a complete revolu- 
tion about the earth in four weeks ; twice during this time — at new 
and full moon— the attraction of the sun and of the moon combine to 



132 



PHYSICAL GEOGRAPHY. 



EIRST-t ;-QUARTER 




N 



Spring If. \\ Spring ,--\ 

— - — *~\ -EARTH-— --,-— \ ';- 

Tide A i !// Tide V 

MOON V V I ,./• MOON 

Neap ! Tide j 



Fig- 53- 

produce an unusually high tidal wave called the spring tide. At 
two other points in the moon's orbit — first and third quarters — the 
crest of the solar occurs in the trough of the lunar tidal wave, the 
combination resulting in an unusually low tidal wave, called the 
neap tide. The weekly change in the height of tidal waves is : 

Head of Bay of Fundy, Spring Tide, 50 feet. Neap Tide, 24 feet. 

Boston, Mass., ... " n.3" " 8.5 

New York, N..Y., . . " 5.4" " 3.4 

Cape May, N. J., . . " 6.0 " " 4.3 

Cape Hatteras, N. C, . " 2.2 " " 1.8 

Savannah Entrance, Ga., . 8.0 " 5.9 

Cape Florida, Fla., . . " 1.8" 1.2 

San Francisco, Cal., . . 4.3 " 2.8 

Astoria, Oregon, . . 7.4 " " 4.6 

Establishment of the Port. — The currents moving 
under the attraction of the moon to form the crest of the 
tidal wave, or high tide, are prevented by their momen- 
tum from stopping immediately when under the attracting 
body. Besides this, the point under that body is con- 
stantly advancing westward on account of the earth's ro- 
tation ; hence, the crest of the wave is always east of the 
meridian under the moon. The momentum depends upon 
the speed of the water and its amount, i. e. , upon the depth 
and shape of the bottom. Hence, though the interval of 
time is always the same between the passage of the moon 



WAVES AND TIDES. 1 33 

over a given locality and the arrival of high tide, this 
interval varies at different localities. It can only be found 
by observation, and when found is called the establisliment 
of the port. 

Thus, the establishment of the port of Boston is n hours, 27 
minutes ; of New York, 8 hours, 13 minutes ; of Cape May, 8 hours, 
33 minutes; of Washington, 7 hours, 44 minutes ; of Cape Hatteras, 
7 hours, 4 minutes ; of Savannah Entrance, 7 hours, 20 minutes. 
Each indicates the time intervening between the moon's transit at 
that point and the arrival of the succeeding high tide. 

Diurnal Inequality of the Tides. — The inclination 
of the earth's axis to the orbit of the moon tends to pro- 
duce a periodical inequality in the heights of the two daily 
tide waves. This is called the diurnal inequality. Every 
two weeks, when the moon is over the equator, both waves 
are of equal height, but in the intervening time one wave 
tends to become higher than the other, and the wave that 
is highest during the fortnight that the moon is north of 
the equator is lowest during the following fortnight when 
the moon is south of the equator. Owing to the shape 
of the Atlantic, the effect of the diurnal inequality is not 
very perceptible in that ocean, but it is a marked feature 
on the Pacific Coast, and its effect is seen in the Gulf of 
Mexico, where it causes only a single tide a day to be per- 
ceptible, and that only at the times when the moon is 
some distance north or south of the equator. 

Fig. 54 (page 134) shows the record, during a fortnight, of tide 
guages at San Francisco and the mouth of the Mississippi River 
respectively. Each of the vertical spaces represents one day, while 
the rise and fall of the tide is represented by the curved lines. 
It is seen that at San Francisco there are two tides each day, but 
one of them is considerably higher than the other, excepting about 
the time when the moon is over the equator. The wave (indi- 
cated by dots in the diagram) which is highest when the moon 
is north of the equator, is lowest when the moon is south. The 
semi-diurnal tides of the Atlantic approach the Gulf of Mexico 



134 



PHYSICAL GEOGRAPHY. 



by the two channels on either side of Cuba. The shape and depth 
of these channels is such that the tide wave travels through them at 
unequal speed. Thus, the crest of the wave from one channel, and 
the trough of the wave from the other, enter the Gulf simultane- 
ously. They therefore neutralize each other, except when the moon 
is far from the equator, and the diurnal inequality makes the waves 
of unequal heights. At such times this difference of height is prop- 
agated through the Gulf as a small diurnal wave, while, when the 
moon is near the equator, no tides are perceptible. 



MOON FARTHEST 
SOUTH OF EQUATOR 

S Si M 




Fig- 54- 



Tides in Lakes and Landlocked Seas. — As the 

force which produces the tides is universal, it affects all 
water surfaces on the face of the earth, but in even the 
largest sheets of water completely cut off from the sea, as 
the Caspian Sea and Lake Superior, the length of the 
water surface is so insignificant in comparison with the 
length of the tidal wave (half the circumference of the 
earth) that the variation of level in the small part of the 
wave formed in them is quite imperceptible. 

Even in the long Mediterranean, the height of the tidal wave on 
open coasts is only 3 or 4 inches, and is generally obliterated by the 
wind. The converging shores render the tides more perceptible 
near the head of the Adriatic and in the Straits of Messina, where 
the eddies and currents "between Scylla and Charybdis" resemble 
those of Hellgate, New York. 



CHAPTER X. 

CURRENTS AND DEPOSITS. 

They that go down to the sea in ships, that do business in great waters ; these 
see the works of the Lord, and his wonders in the deep — Psalm cvii : 23, 24. 

Currents. — In addition to the forward and back move- 
ment of the water in wind and tidal waves, each ocean is 
traversed by systems of true currents, or continuous move- 
ments of the water in the same direction. Several causes 
combine to produce these continuous currents; the princi- 
pal cause, however, is the inequality in the density of the 
water in different parts of the sea, arising from differences 
in temperature and saltness. 

Effect of Temperature. — Since water expands and 
becomes less dense when heated, the surface of the sea 
stands somewhat higher near the tropics than in the frigid 
zones, where the water is 40 or 50 colder. Gravity 
gives the surface water a tendency to flow down the gentle 
slope thus formed, from the tropics toward the nearest 
pole, while the increased pressure upon the deeper water 
caused by the arrival in higher latitudes of the surface 
water from the tropics, gives the deeper water a tendency 
to flow back toward the tropics. These movements are 
facilitated by the fact that the deep polar water is more 
salty, colder, and hence heavier than the deep water of 
lower latitudes, while the polar surface water, though 
colder than the surface water at the tropics, is not so 
dense or heavy, because the melting ice renders it less 
salty. 

(135) 



136 PHYSICAL GEOGRAPHY. 

Effect of Saltness. — The constant trade winds start 
from the tropics very dry, but arrive at the equatorial 
calms saturated with vapor. Rising over these calms, the 
vapor condenses and causes almost constant rains. Since 
vapor is perfectly fresh, the region of the sea from which 
it is taken is left more salty, and the region on which it is 
precipitated as rain is made less so ; hence, the surface 
water near the tropics is more salty and heavier than that 
in the equatorial calms. The tropical surface water, heavy 
through its excess of salt, can not sink far because the 
deeper water is equally heavy from its lower temperature ; 
it consequently moves toward the equatorial calms to dis- 
place the lighter water there. 

Thus, the varying temperature and saltness of the 
water in different parts of the sea give the surface water a 
general tendency to move toward the equator in the torrid 
zone, and toward the poles in the temperate and frigid 
zones ; and to the deeper water, in all zones, a general ten- 
dency to move toward the equator. 

The direction in which ocean currents flow is greatly 
modified, however, by (1) the rotation of the earth; (2) 
the configuration of the coast and sea bottom; (3) by 
other currents ; and (4) by the winds. 

The rotation of the earth gives to moving water, as it does to mov- 
ing air (page 79), a tendency to turn out of a straight course. In the 
northern hemisphere it tends to turn to the right, and in the southern 
hemisphere to the left. This tendency affects moving water at all 
depths, and increases from the equator to the poles. 

The coast of an ocean deflects currents at all depths which flow 
against it. If the current strikes the shore .almost at right angles, 
part of it is deflected to the right and part to the left. The config- 
uration of the sea bottom influences the direction of deep water 
currents in the same way, for as the heaviest water sinks to the bot- 
tom, this water, when moving as a current, can not rise through the 
lighter water above to pass over submarine banks or ridges, which 
therefore deflect currents in the deeper water. 



CURRENTS AND DEPOSITS. 1 37 

A current meeting another at any angle deflects it, and is itself 
deflected to the right or left, or in both directions, according to the 
angle of meeting and the respective strength of the currents. 

The friction of the wind on the sea surface tends to move the 
wate in the direction of the wind. If the wind moves in the same 
direction as the current, it tends to make the current move faster ; if 
it blows obliquely across the current, it tends to deflect the current ; 
if it blows against the current, it tends to check and may even re- 
verse for the time being a gentle flow. The effect of the friction of 
the wind is always superficial, however ; Professor Ferrel estimates 
that its influence immediately beneath the surface is less than xoo tn 
of that arising from unequal density of the water. 

Surface Currents. — In consequence of these modify- 
ing influences, the general movement of the surface water 
in each of the oceans is outward and around the tropical 
regions where the surface water is densest and heaviest, 
the currents thus forming great outward moving whirls 
similar to anticyclones in the atmosphere. The general 
movement of these whirls is westward on either side of 
the equatorial calms, away from the equator in the western 
part of the oceans, eastward between latitudes 40 and 
6o°, and toward the equator in the eastern part of the 
oceans. In the narrow region of equatorial calms a 
"counter-current" moves eastward in all the oceans. Be- 
yond latitude 50 , in the southern hemisphere, there is 
but little land to deflect the movement of the surface water 
from its general easterly course around the globe. In the 
higher latitudes of the northern oceans, however, and es- 
pecially in the Atlantic, with its Arctic extension, the 
easterly moving water between 50 and 6o° latitude en- 
counters the west coasts of the continents. Part of it is 
deflected northwardly along these coasts, thus causing a 
southward return current along the east coasts of the con- 
tinents in these higher latitudes. 

While the movement of surface currents is generally similar in 
all the oceans, differences in the shape and extent of the coast-line, 



Sp 



RELATIVE DENSITY 

of the Surface Water, 

THE SURFACE CURRENTS 

and Equatorial limits of 

FLOATING ICE 



^J 







SllL ( l_®. , !S^!i^.i?|i_t!l?;2 ' '_° 2 _5_ Thearro ws _f[y wilh_t_ h 5- c — L r —— 



X 



" 1.025 to 1.026 

" 1.026 to 1.027 

" 1.02k to 1.028 

" mor.e than 1.028 



A square mile of water r foot deep 
with a Sp. Gr. of 1.027 ,s about 851 
tons heavier than an equal volume 
with a|Sp. Gr. of i.02|6. 



(138) 




(139) 



I4O PHYSICAL GEOGRAPHY. 

and in other modifying influences, cause local peculiarities in the 
speed, temperature, and constancy of the currents in each ocean, and 
these vary at different seasons of the year. 

The Gulf Stream. — The western part of the tropical whirl in the 
Atlantic is called the Gulf Stream, and off the southern coast of 
Florida it is made one of the most rapid of ocean currents by the 
peculiar configuration of the coast in that vicinity. The equatorial 
current enters the Gulf of Mexico through the broad, deep 
Yucatan Channel, and forces an equal amount of water to flow 
out through the Strait of Florida. This strait being relatively 
shallow and narrow, the outflow is more rapid than the inflow. 

The Kuro Siwo, or Black Stream, as the corresponding current 
in the western part of the Pacific is called, though a well marked 
current, is not so strong or well marked as the Gulf Stream because 
of the chain of islands which border the east Asiatic coast — For- 
mosa, Japan, etc., — among which part of the current is deflected 
from its regular north-easterly course, and also because of the 
strong winter monsoon of that region, which at that season diverts 
part of this current to the south-west through the Malay Archipelago 
into the Indian Ocean. 

In the North Indian Ocean the effect of the monsoons upon the 
surface currents is very marked. In January the north-east mon- 
soon strikes the northern part of this ocean as a dry land wind, and 
evaporating water rapidly, renders the surface water salty and 
heavy. Aided by the friction of the wind, the water flows south- 
westward toward the lighter and fresher water in the region of 
equatorial calms and rains. In July, however, the south-west mon- 
soon, rendered damp near the equator, pours down fresh water on 
the northern part of the ocean, and the Salter and heavier equatorial 
water, aided by the friction of the winds, flows north-eastward and 
along the coast into the China Sea to augment the strength of the 
Kuro Siwo. 

Effect of Surface Currents upon Temperature. — 

The water composing the surface currents is warmed in 
equatorial regions, and arrives in higher latitudes with a 
higher temperature than the sun is able to maintain at that 
latitude. It therefore cools by imparting its excess of heat 
to the water below and the air above, and arrives at the 
equator again, in the eastern part of the oceans, cooler 



CURRENTS AND DEPOSITS. 



141 



than the equatorial air and water. These are slightly 
cooled by imparting heat to the cooler current, and aiding 
the sun to warm it again during its westward flow across 
the ocean. Thus, all currents tend to moderate the tem- 
perature of the region they traverse ; if they come from a 
warmer region, they tend to raise the temperature ; if 
from a colder region, to lower it. It has already been 
stated (page 61) that about one half of the heat received 
by the whole torrid zone is carried by ocean currents into 
colder latitudes. 



west ATLANTIC OCEAN 

60°W. 50 ( 'W. 40 C W. 




SURFACE 
00 FATH 



PACIFIC OCEAN 



140 E.LONQ. 




Fig. 55- 



Since the currents between the equator and about 45 latitude 
move from the equator in the western part of the oceans and 
toward the equator in the eastern part, it follows that the western 
part of the oceans in these latitudes contains a greater amount of 
warm water than the eastern part. This is well shown in the tem- 
perature sections across the Atlantic and Pacific oceans along the 
parallel of 35 north latitude (Fig. 55). A temperature higher than 59 
extends to a depth of over 300 fathoms in the western part of the 
Atlantic, but to scarcely 100 fathoms in the eastern part. Owing to 
the less relative strength of the warm current in the Pacific (the 
Kuro Siwo), and the greater size of that ocean, the isotherm of 59 
lies at a less depth in the Pacific than in the Atlantic ; but while it 
reaches a depth of nearly 1 50 fathoms in the western part of the 
ocean, it rises to within less than 50 fathoms in the eastern part. In 
higher latitudes, on account of the reversed directions of the cur- 



142 PHYSICAL GEOGRAPHY. 

rents, the waters of the north Atlantic and Pacific are warmer in the 
eastern than in the western parts of these oceans. In the Atlantic a 
narrow branch of the cold polar current follows the American coast 
southward to the latitude of the Carolinas, forming a "cold wall" 
of water between the warm Gulf Stream and the shore. The cold 
southwardly flowing current in the high latitudes of the western 
Atlantic brings vast numbers of icebergs down to the neighborhood 
of Newfoundland, where, meeting the warmer waters of the Gulf 
Stream, the bergs melt rapidly and deposit their load of rocky 
material. This deposit, gathering through untold ages, has pro- 
duced the shoals of that vicinity called the Newfoundland Banks. 

Deep Sea Currents. — The systems of surface currents 
affect but a comparatively thin layer of water. They 
seldom extend to a greater depth than a few hundred 
fathoms. The movements of the great mass of sea- water 
below this depth can not be directly observed, but pro- 
gressive movements or currents in it are known to exist 
because of its temperature. 

Only a comparatively thin layer of surface water is directly 
affected by the heat of the sun ; this water, however, affects the 
temperature of the layer beneath, and this of the still deeper water ; 
hence, if there were no currents the temperature of the sea in any 
latitude would eventually become uniform at all depths ; that is, it 
would be about 8o° from surface to bottom at the equator, and about 
29 in polar regions. The fact that at greater depths than 800 
fathoms the water, even under the equator, never has a temper- 
ature higher than 40 , proves that the deeper water is constantly 
being replaced by cold water before it can be warmed by the warmer 
water above. Now, from the equator nearly to the polar circles the 
temperature of the surface water is higher than 40 (see Fig. 46), 
and tends to raise the temperature of the deeper water above that 
point ; hence, the deep water that arrives in equatorial latitudes with 
a temperature less than 40 must come from the frigid zones. 

Direction and Velocity of Deep Sea Currents. — 

The configuration of the coasts and bottom of the oceans 
indicates that by far the larger part of the cold deep sea 
currents, even in the northern hemisphere, comes from the 
Antarctic Ocean ; but they must move with extreme slow- 



CURRENTS AND DEPOSITS. 



H3 



ness, otherwise the finely powdered material which com- 
poses the bottom of the ocean would be swept away by 
them. 

The only places where the Arctic water could flow south are be- 
tween Europe and America into the Atlantic, and through Behring 
Strait into the Pacific. But a submarine ridge, on which Iceland 
stands, extends entirely from Europe to America. It rises every- 
where to within 500 fathoms of the surface, and therefore prevents 
the deeper water of the Arctic from entering the Atlantic. East of 
Iceland all the water above the top of the ridge is warmer than 40 , 
and moves northwardly ; hence, it is only in the comparatively 
narrow channels between Iceland and Labrador, and in the entirely 
insignificant Bering Strait, that any of the cold surface water of the 
Arctic escapes southwardly. 

Theory Confirmed. — The theory that the low temper- 
ature of the deeper water in the sea is produced by cold 
under-currents from the polar regions, is confirmed by the 
comparatively high temperature of the deeper water in 
regions which such under-currents can not enter on account 
of an intervening submarine ridge. 
The Mediterranean and Caribbean 
seas are such regions, as well as 
several of the east Asiatic seas, and 
perhaps the whole north Pacific 
Ocean. 

As indicated in Fig. 56, the tempera- 
ture of the Atlantic opposite the Strait of 
Gibraltar falls continuously from over 
70° at the surface to about 37° at the 
bottom in 2,200- fathoms. The Strait of 
Gibraltar, with a depth of less than 200 
fathoms, admits to the Mediterranean no water colder than 55 . 
The deep basin of the Mediterranean is thus filled with water no 
colder than that which enters in the lower part of the inlet current 
from the Atlantic with a temperature of 55 . Hence, the tempera- 
ture of the Mediterranean falls continuously only from the surface 
to the depth of the bottom of the inlet current (about 125 fathoms), 




Fig. 56. 



144 PHYSICAL GEOGRAPHY. 

beyond which, clear to the bottom (over 2,000 fathoms in some 
places), the temperature of the water is uniform at 55 . A uniform 
temperature of 39>2° was long since observed in the Caribbean Sea 
and the Gulf of Mexico at all depths greater than about 1,000 
fathoms. The existence of a channel to the Atlantic, with a depth 
of about 1,000 fathoms, was therefore inferred, although all known 
channels were much shallower. In 1884, however, a channel be- 
tween Puerto Rico and Santa Cruz was discovered having a depth 
of 926 fathoms, and a bottom temperature of 39^2 °- The deep seas 
of the Malay Archipelago have uniform temperatures below depths 
varying from 400 to 900 fathoms, from which it is inferred that each 
of these seas is inclosed by a submarine ridge, whose lowest point 
corresponds to the depth at which the uniform temperature begins. 
All temperature observations in the Pacific north of a line from 
northern Chile to China, indicate a uniform temperature below a 
depth of 1,500 fathoms, while to the south of this line the tempera- 
ture decreases constantly to the bottom. Hence the inference that 
a submarine ridge, rising to within at least 1,500 fathoms of the sur- 
face, unites South America with Asia, and prevents the cold bottom 
water of the Antarctic from entering the North Pacific. 

Currents between the open ocean, and nearly in- 
closed arms of the sea, depend to a great extent, like the 
surface-currents in the open ocean, upon evaporation and 
precipitation. If more water falls in the basin of the par- 
tially inclosed sea than is evaporated from, its surface — as 
in New York Bay and the Baltic and Black seas — its sur- 
face tends to rise higher than that of the ocean, and a 
current from the bay or sea into the ocean is the result. 
If, however, less water is precipitated in the basin of the 
sea than is evaporated from its surface, as the Mediter- 
ranean and Red seas, the surface tends 'to fall below the 
ocean level, and a current from the ocean into the sea is 
the result. 

In the latter case, since only fresh water is removed by evapora- 
tion, and since the level of the inclosed sea is maintained by a flow 
of salt water from the ocean, the tendency is for the sea to become 
constantly more salty. Both the Mediterranean and Red seas are 
more salty than the ocean, but their water does not appear to in- 



CURRENTS AND DEPOSITS. 



H5 



crease in saltness. The tendency to increase in saltness must there- 
fore be counteracted by a current which carries just enough of the 
excessively salty water of the inclosed sea into the ocean to prevent 
a constant increase of saltness. As the inclosed sea water is heavier 
than the fresher ocean water, the outflowing current occupies the 
bottom, and the inflowing current the top of the channel by which 
the sea communicates with the ocean. Therefore, the uniform tem- 
perature in the Mediterranean begins, not at the depth of the Strait 
of Gibraltar, but at the depth of the bottom of the inflowing current ; 
for the outflowing under-current is as effectual a barrier to the en- 
trance of colder ocean water as the submarine ridge itself. 

Deposits of the Sea. — In addition to the solid matter 
dissolved in sea-water, which gives it the salty and bitter 
taste, there is always a quantity of solid matter, in coarser 
or finer grains, which is gradually sinking through sea-water, 
and forming a deposit on the bottom. This sediment is 
derived chiefly from three sources: (1) the continents, (2) 
the animals and plants which inhabit the sea, and (3) the 
material ejected from volcanoes. According as the bot- 
tom in any region of the sea is composed principally of 
matter derived from one or other of these sources, the de- 
posit is called continental, organic, or red clay. 

Continental Deposits cover about two fifths of the 
sea bottom. They form the sea bottom for a distance of 
300 or 400 miles from the shores of all 
the continents and continental islands, 
and extend completely across the polar 
oceans and all partially inclosed seas. 
They consist of variously colored muds, 
composed principally of very minute 
rounded fragments of the rocks which 
constitute the land. These muds also Fmj-57- 

contain organic remains and volcanic minerals. 

Fragments from continental deposit, magnified ten times, are 
shown in Fig. 57. Pieces of the rocky coast are being constantly 
broken off and ground to powder by the force of the waves, while 






MS 9 



'o>> <5>W<s. 









:m*t 



146 



PHYSICAL GEOGRAPHY. 






mBL- 



.:*-■-•.■ - • - 

mm: - 



- - 










Fig. 58.— Globigerina Ooze. 
(Magnified 13 times.) 



Fig. 59. — Pteropod Ooze. 
(Magnified ■$ times.) 



the water of every stream is more or less muddy, according as its 
current is carrying or rolling along a greater or less amount of 
rocky, earthy, or other continental material. The larger fragments 
broken off by the waves or brought down by the rivers, sink to the 
bottom near the shore of the ocean, to be rolled about and ground 
finer by the waves', but the finer pieces sink more slowly, and are 
carried farther away by the ocean currents. 
It is only in exceptional cases, however, such 
as floating ice, etc., that even the most 
minute fragments are carried more than 
300 or 400 miles before they settle to the 
bottom. 




Fig. 60. 

Radiolaria Ooze. 

(Magnified 50 times.) 



Organic Deposits differ from con- 
tinental deposits in containing no re- 
mains of continental rocks. An organic 
deposit constitutes the bottom in such 
portions of the sea as lie beyond the 
limits of the continental deposits, and 
have a depth less than 2,900 fathoms. 
It is a soft, fine mud, or ooze, composed 
principally of the shells or stony frame- 
work of minute organisms which live 
near the surface of torrid and temper- 
ate seas. It is called globigerina, pteropod, 
or radiolaria ooze, if the shells of these 
animals respectively are most numerous, or diatom ooze if 
the stony frustules of this plant are in excess. 




Fig. 61. — Diatom Ooze. 

(Magnified ioo times.) 



Currents and deposits; 



147 



*> S J# cf.4>'-: rv : ; t*w ' 







Fig. 62. 



The shells or stony frame-work of sea organisms are largely com- 
posed of carbonate of lime, extracted from sea-water during the life 
of the organism ; at death these stony structures begin to sink, and 
are slowly dissolved again by the sea-water. If the sea is deeper 
than about 2,900 fathoms, the calcareous or limy portions are entirely 
dissolved before they reach the bottom ; but if 
of less depth, fragments of them may reach 
the bottom and be covered up and protected 
by following pieces. Hence, organic deposits 
are always calcareous, sometimes being nearly 
pure carbonate of lime. 

Red Clay Deposits differ from con- 
tinental and organic deposits in the 
general absence of continental debris 
and calcareous organic remains. The red clay covers the 
sea bottom beyond the limit of the continental deposits, in 
depths greater than 2,900 fathoms. It is a stiff clay, 
greasy to the touch, plastic when wet, but very hard when 
dry. It is composed almost exclusively of 
the minerals which are found in volcanic 
rocks. Most of the minute mineral frag- 
ments found in it are sharp and angular, (Fig. 
62, magnified 100 times,) in marked contrast 
Fig. 63. to t k e rounc ied fragments of the continental 

deposits. The surface of the deposit is strewn with pieces 
of pumice stone, minute particles of magnetic iron of 
meteoric origin, and with great numbers of the hardest 
bones of sea animals, as the ear-bones of whales and the 
teeth of sharks, some of which belong 
to species once plentiful but long since 
extinct. The older bones are covered 
with a thick coating of oxide of manga- 
nese, while the bones of more recent 
species are quite clean. Figure 63 
shows the incrusted tooth of a shark, Fi s- 64. 

and Figure 64 the incrusted ear-bone of a whale. 





I48 PHYSICAL GEOGRAPHY. 

The volcanic materials which compose the red clay are derived 
from pumice stone, which is so light that it floats for great distances 
before sinking ; from volcanic ashes, which are carried to great dis- 
tances by the winds ; and from volcanic lavas and tufas laid down 
directly on the sea bottom. All these volcanic products are rich in 
the minerals of which clay is composed, and these minerals, being 
liberated by the chemical action of the sea-water, reunite in the pro- 
portions to form the red clay. 

What the Deposits Teach. — The character of the 
various deposits goes far toward confirming the belief that 
the present ocean basins have been depressed regions, and 
that the present continents have been elevated regions con- 
tinuously, from a very early period of the earth's history ; 
but while the present regions of organic and red clay 
deposits have always been covered by water, the marginal 
region of continental deposit, as well as the present land, 
have been subjected to many upward and downward move- 
ments, by which large areas of each have been alternately 
raised above and lowered beneath the surface of the sea. 
Thus, the continents and the oceans, though constantly 
varying somewhat in shape and size, have always main- 
tained their present general arrangement. 

The great antiquity of the red clay deposit, and the extreme slow- 
ness with which it collects, are indicated by the abundance of me- 
teoric fragments, and whales' and sharks' bones, many of them of 
extinct species and deeply incrusted, which are found on the surface 
of this deposit. Great numbers of fragments and bones probably 
settle upon the other deposits also, but are covered up and buried in 
the more rapidly accumulating continental and organic debris. Most 
of the rocks of the continents bear evidence of being a hardened sea 
deposit very similar to the continental deposits now forming, but no 
rocks have been found similar to organic and red clay deposits of 
the deep open ocean. From this it is inferred that most of the conti- 
nental rocks were formed as a continental deposit beneath the sur- 
face of the sea like the present continental deposits, at no great 
distance from the land, and afterward elevated above sea-level. 
Such gradual elevation or subsidence of coast regions is now in 
actual progress in many parts of the earth. 



PART IV.— THE LAND. 



CHAPTER XL 

DIVISIONS OF THE LAND. 

And God said, Let the waters under the heaven be gathered together unto one 
place, and let the dry land appear: and it was so. — Genesis i: 9. 

Comparative Smoothness of the Earth's Surface. — 

In speaking of the earth as a whole, its solid surface was 
considered as being perfectly smooth, and in comparison 
with the vast dimensions of the planet, the irregularities 
of its surface are insignificant. These irregularities are of 
vast importance, however, since they cause the division of 
the surface of the earth into areas of sea and land. 

The relative insignificance of the surface irregularities can be 
appreciated from the diagram on the next page (Fig. 65), in which 
the heights and depths have been exaggerated ten times. 

The Land. — The tops of the highest irregularities on 
the earth's surface protrude above the surface of the sea 
and form land. The total land area of the world is about 
52,500,000 square miles, and constitutes but little more 
than one fourth {26%%) of the surface of the planet. 

The Level of the Sea. — The sea has a smoother sur- 
face than the solid globe. Though always slightly rough- 
ened by waves, it never varies more than a few feet from 
perfect smoothness. Its mean height when half-way be- 
tween low and high tide is usually adopted as the base, 

P.G.-9. (x 49 ) 



i5o 



PHYSICAL GEOGRAPHY. 




Fig. 65. — The Proportional Roughness of the Earth's Surface Exaggerated 
Ten Times. 

called sea-level, from which all differences of elevation in 
the earth's solid surface are measured. 

Regions of Elevation and Depression. — The mean 
height of the land above sea-level is a little less than one 
half a mile. As the mean depth of the sea is 2*4 miles, 
the total mean height of the land above the sea floor is 
about 3 miles. An elevation half as great (that is, 1^ 
miles above the sea floor), may therefore be taken to 
divide the regions of elevation in the earth's crust from 
the regions of depression. In other words, not only the 
land, but all parts of the sea bottom on which the water 
is less than 1 mile deep, are to be considered as regions 
of elevation, while only the sea bottom at greater depths 
is to be considered the region of depression. This 
region of depression is shown in solid black in the map 
on pages 152 and 153; the regions of elevation are 
shaded or are left white. 

Region of Elevation. — The map shows that there is 
but one great region of elevation. It extends entirely 
across the northern hemisphere, and at three places pene- 



DIVISIONS OF THE LAND. 151 

rrates the southern hemisphere to about 40 south lati- 
tude. The height of this continuous region of elevation 
is not uniform ; at certain localities it does not reach quite 
to the level of the sea, but enough of it protrudes above 
the sea to constitute almost all vrWoths) °f the land on the 
globe. It may therefore be called the continental plateau. 
The only other regions of elevation rise in small, isolated 
areas in various localities, the largest being about the south 
pole, and in the tropical Pacific Ocean. Collectively, these 
isolated regions of elevation form but x 7 Q ths of the land 
on the globe. 

The primary cause of the elevation of the conti- 
nental plateau is not yet known. It seems probable that 
the part of the earth's crust forming this region is lighter, 
bulk for bulk, than the part beneath the deep sea. This 
of itself would probably cause the former to be a region 
of elevation. As explained on page 42, the earth's crust 
at a depth of a few miles probably behaves as if it were 
plastic or liquid, if the pressures on adjacent portions of it are 
very unequal, the rock particles moving or ' ' flowing ' ' side- 
ways from under the region of greater pressure, until, by 
this transfer of matter, the weight and pressure become 
uniform. When the weight thus becomes uniform, the 
lines of equal pressure would be level. But to produce 
equal pressures, the lighter part of the crust would have 
to be thicker than the heavier part; hence its upper sur- 
face would be further above the level pressure lines below, 
and would form a region of elevation. The plateau crust 
may be composed of lighter rock than the crust of the sea 
bottom, or it may be lighter because it is hotter and more 
expanded; but science can not yet satisfactorily explain 
why either should be the case. 

In shape, the plateau is roughly curved, like an ir- 
regular horseshoe; the toe lies in the arctic regions, and 



REGIONS OF ELEVATION 

EXPLANATION. 




(15*) 



AND DEPRESSION 



EXPLANATION. 

•More than 6000 feet below Sea level 
HH!~ Less ,, 

Less ,, 2000 
More ,, 
A B=Axis main continental plateau. 




U33) 



154 PHYSICAL GEOGRAPHY. 

the two arms extend into trie southern hemisphere. The 
line AB (pages 152, 153) may be regarded as the curved 
axis of the main portion of the plateau. The deep pocket 
formed in the concavity of the curve is the basin of the 
north Atlantic. From the outer side of the main plateau 
a small third arm extends into the southern hemisphere, 
and separates the basins of the Pacific and Indian oceans. 
The axis of this third arm is shown by the dotted line 
CD. 

Elevation and Coast-line. — Not only is the greater 
part of the plateau sufficiently elevated to protrude above 
the sea to form land, but the highest part, indicated by 
the unshaded portion of the map, forms an almost contin- 
uous tract along the outside or convex margins, while the 
concave margins are generally low, being broken only by 
isolated highland regions. The convex sides of the 
plateau are therefore steep, and possess a very regular 
coast-line, while the concave side has a gentle slope, oc- 
cupying the greater part of the width of the plateau, and 
continues beneath the sea, fringing that side with a greater 
width of shallow water. The coast-line of the low, con- 
cave margin is made very irregular by several deep in- 
dentations which admit the sea far on the plateau to form 
great continental seas. The largest of these are the 
Arctic Ocean, the Mexican-Caribbean Sea, the Mediter- 
ranean, and the seas of the Malay Archipelago, and they 
are located where the bends of the axis are sharpest. 

Continents. — The depressions occupied by the Arctic 
Ocean and the Malay seas, extend entirely across the 
plateau, breaking through the high, convex margin in 
Bering Strait in the one locality, and in the several nar- 
row straits between the Sunda Islands (Sumatra, Java, 
Timor, etc.), in the other locality. The land of the 
plateau is thus separated into three great, continuous 



DIVISIONS OF THE LAND. 1 55 

masses, or continents, and numerous smaller, isolated 
masses, or islands. The three continents collectively con- 
tain more than 92% of the land on the globe. The 
islands rising from the continental plateau are distinguished 
as continental islands ; collectively, they comprise almost 
7% of the land on the globe. The continents are very 
unequal in size ; the largest, or Eastern Continent, con- 
tains about 59% of all the land; the next in size, or the 
Western Continent, almost 28% ; and the smallest, or the 
Australian (southern) Continent, is the only one lying en- 
tirely in the southern hemisphere, and contains less than 
6% of the land on the globe. 

TOTAL I ANn 1 I 

EASTERN (mainland) fflWKffffffMIFWBfflBFlPllllltllilllllllWHfflTrTW 

WESTERN „ miiiM ■ iiim«i 

AUSTRALIAN t) — '. Hi 

CONTINENTAL ISLANDS . HHI 

OCEANIC ISLANDS „__ 1 

UNITED STATES I 1 

Fig. 66. — Relative Areas of Continents and Islands. 

Grand Divisions. — The depression of the Mexican- 
Caribbean Sea penetrates to the narrow highland margin 
on the convex side of the plateau, and determines the 
two natural and nearly equal grand divisions of the 
Western Continent — North America and South America. 
A corresponding depression in the opposite arm of the 
main plateau is occupied by the Mediterranean, and con- 
tinues across the plateau in the narrow and gorge-like de- 
pression occupied by the Red Sea. The heads of these 
seas almost meet at the narrow Isthmus of Suez, and thus 
nearly detach one third of the Eastern Continent from the 
rest to form the natural grand division — Africa. The re- 
mainder of the Eastern Continent strictly forms a single 
natural grand division — Euro- Asia. 



156 



PHYSICAL GEOGRAPHY. 



Before the extent of the Black and Caspian seas was accurately 
known, their depressions were supposed to divide this grand division 
into two parts, which were named Asia and Europe. The error was 
subsequently discovered, but the names remained, and the Eastern 
Continent is still said to be composed of three grand divisions, 
though Europe occupies but little more than one tenth of its area, and 
the boundary between Europe and Asia is arbitrary rather than real. 
Each of these five grand divisions is frequently though wrongly 
called a continent. 

Distribution of Continental Islands. — More than 
85% of the area of continental islands occurs in the great 



1 — — — ' t^t - " 

A • S 1 A -< / " 4U 

~\ F\ Q I / JS ^. -^ FORMOSA 



LAND 




Fig. 67. — Continental Plateau between Asia and Australia. 

bends of the continental plateau ; thus, almost one half 
(46%) occurs in the Arctic Ocean, and by far the largest 
part of this island area, including Greenland, Iceland, and 
Great Britain, occurs on the concave margin, or rim, of the 
plateau (see chart, pages 152, 153). More than one third 
(36%) of the continental island area occurs in the great 
bend of the Australian arm of the plateau, where it forms 
the Malay Archipelago (Fig. 6j) and the continuous chain 
of islands along the concave margin of the bend, of which 



DIVISIONS OF THE LAND. 1 57 

Japan, the Philippines, and New Guinea are the principal 
groups. More than 3_% of the continental island area 
occurs in the minor bends of the plateau occupied by the 
Caribbean and Mediterranean seas, the islands in the 
former locality occurring along the concave rim of the 
bend as the chains of the West Indies (see chart, pages 
152, 153)- 

The remaining 15% of the continental island area embraces 
islands which occur along the margins, but are not confined to the 
concave margin of the plateau. About two fifths of this area com- 
pose islands lying close to the continents, and well within the 
limits of the plateau, as Newfoundland, Tasmania, and Ceylon, and 
the Alaskan and Chilean islands. The remaining three fifths com- 
pose the two groups of large islands — Madagascar and New Zea- 
land. These are somewhat exceptional among continental islands, 
because they occupy outlying spurs, almost if not quite detached 
from the continental plateau, and because many of the forms of life 
on these islands differ from those of the adjacent continent. These 
islands are properly classed as continental islands, however, since 
their geological structure and some of their forms of life correspond 
to those of the adjacent continent, and because the water which sep- 
arates them from the continent is shallow in comparison with that on 
the opposite or oceanic side of the islands. 

Oceanic Islands. — About youths of the land on the 
globe occurs in numerous very small masses in the midst 
of the oceans and far from the continents. They occur in 
each of the three great oceans, but are most numerous in 
the tropical Pacific, where they lie in long, nearly straight, 
or gently curving lines extending in a general north-west 
and south-east direction. They contain none of the kinds 
of rock which compose the greater part of the great land 
masses, and, unlike all the continents and continental 
islands, they contain no native four-footed animals. 

These islands are thought to be the tops of volcanic cones which 
have built themselves up from great depths by the solidification of 
successive outflows of melted rock or lava around some aperture in 
the earth's crust. They generally rise from the crest of the low sub- 



i58 



PHYSICAL GEOGRAPHY. 




Coral Formations. 



marine ridges or plateaus which traverse the ocean basins, which 
accounts for the lineal arrangement of the oceanic island groups. 
The submarine ridges are probably formed in the same general 
manner as the continental plateau, — by differences in the temperature 
and density of adjacent regions of the earth's crust. These differ- 
ences, however, are probably relatively slight, hence the submarine 
ridges do not stand so high as the continental plateau. Being larger, 
the Pacific contains a greater number of ridges than other oceans. 
The outflows of lava which largely compose oceanic islands, are 



DIVISIONS OF THE LAND. 1 59 

probably the direct result of the fracturings of the earth's crust and 
the heat generated by these movements of upheaval. 

Coral Islands and Reefs. ^-In the shallow water about 
the shores of many oceanic islands, and in fact of all 
coasts where the water is warm and clear, low, rocky reefs 
frequently occur. These rise to about the level of low 
tide, and are composed of the peculiar coral limestone. 

Some oceanic islands rising from great depths seem to 
be composed entirely of this limestone. Such islands 
never rise more than io or 12 feet above sea-level, and 
usually take the form of a narrow strip or ring of rocky 
land, wholly or partially surrounding a shallow lake, or 
lagoon, of sea-water. These islands are called atolls, and 
are common in the warm parts of the Pacific and Indian 
oceans. Although apparently composed entirely of coral 
rock, it is probable that this rock merely covers and con- 
ceals a volcanic foundation at a comparatively slight depth. 

Coral reefs and islands are composed of rock which is nearly 
pure carbonate of lime, and is remarkable in its manner of forma- 
tion. Myriads of sea animals, called polyps, live in 
vast colonies on the bottom of clear, shallow, tropical 
seas. The skeletons of these animals are carbonate 
of lime extracted by the polyps from the sea-water. 
The general cross section of a polyp is shown in 
Fig. 69, the black portion indicating the stony skele- 
ton. As the polyps grow upward, the lower part of 
their cylindrical skeleton becomes a solid stalk or stem of stone, 
from the sides of which other polyps grow outward, thus eventually 
forming an intricate network of stone branches. The surfaces of both 
this network and the parent stem may be covered with living polyps. 
Branches are constantly being broken off and ground into sand by 
the force of the waves, and this sand slowly fills up the spaces be- 
tween the various stems and branches until the whole becomes 
cemented into solid coral reef rock. This also gradually grows by the 
same process, both upward to the surface of the water, and outward 
to a depth of about 20 fathoms, beyond which polyps on its surface 
can not live. If the water and bottom close to the shore are clean, 




l60 PHYSICAL GEOGRAPHY. 

the reef extends quite to the shore, and is called a fringing reef ; 
but if the water and bottom are muddy, a channel of water inter- 
venes between the shore and the reef, and the latter is then called a 
barrier reef. Polyps thrive best in a heavy surf; hence, the outside of 
a barrier reef grows faster than the inside, which contains but few 
live polyps, and often does not grow as fast as it is dissolved away 
by the sea-water. Many islands of the Pacific are almost sur- 
rounded by a barrier reef, separated from the shore by a broad 
channel of water several fathoms deep. Atolls are much like such 
barrier reefs, except that they inclose no island. Since polyps 
thrive only to a depth of 20 fathoms, an atoll can be started only in 
shallow water. Rising as a reef to the surface in such a shallow 
place in the open ocean, it naturally assumes the irregular circular 
shape around a shallow lagoon, for the heavy surf favors the rapid 
growth of the outside edge, while the interior gradually dissolves 
away under the action of the sea-water. The size of the inclosed 
lagoon thus very gradually increases by the seaward growth of the 
encircling reef. Pieces broken from the outer edge of the reef and 
cast up by the waves gradually raise the reef above the surface of 
high tide, while wind and currents bring seeds which take root and 
cover the atoll with vegetation. Larger pieces, broken off by the 
waves, fall to the bottom and form a talus, or slope, of fragments of 
coral rock, on which the living surface portion of the atoll slowly 
advances into deep water. 

Antarctic Lands. — In addition to the known land, an 
indefinite, but probably a comparatively small area of 
land is supposed to occur within the antarctic circle. 
Whether this land area is continuous, or whether it is 
broken up into an island group, is not known ; but as the 
rocks found on the bottom of the southern oceans, and 
which have evidently been dropped by antarctic icebergs, 
resemble the rocks of the known continents and conti- 
nental islands, it is inferred that the antarctic lands should 
be classed with them rather than with the oceanic islands. 



CHAPTER XII. 

THE SURFACE OF THE LAND. 

Go up and view the country. — Joshua vii : 2. 

Average Elevation. — The average elevation of the 
land on the globe is about 2,000 feet above sea-level. 
There is of course land much higher than this in each 
grand division ; but if the entire land surface were reduced 
or increased to a uniform elevation, the resulting level sur- 
face would be about 2,000 feet above the sea. 



Grand Division. 


Average Elevation. 


Highest Elevation. 


Asia 


2,884 feet. 


Mount Everest, 29,002 feet. 


Africa 


i,975 " 


Kilimanjaro, 20,065 " 


North America 


1-954 " 


St. Elias Alps, 19,500 (?) 


South America 


1.764 " 


Aconcagua, 23,910 " 


Australia 


1,189 " 


Clarke, 7,256 " 


Europe 


958 " 


Elbrooz, !8,493 " 



Average of all land, 2,120 feet. 



Lowland and Highland. — Hence, in comparison with 
the land surface of the globe, any land whose surface lies 
at a less elevation than 2,000 feet may be considered as 
lowland, while all land at a greater elevation may be re- 
garded as higJdand. 

The Surface of both highland and lowland is uneven. 

It does not slope uniformly either in rate or in direction, 

(161) 



I 62 PHYSICAL GEOGRAPHY. 

over any considerable area. In consequence of the diver- 
sity of slope, the surface of both lowlands and highlands 
is composed of a series of relatively high regions, sepa- 
rated from each other by a series of relatively low regions. 
These regions are of course high and low only in compar- 
ison with one another, for the low regions of the high- 
lands have a greater elevation above the sea than the high 
regions of the lowlands. 

Mountains and Hills. — A region is usually called a 
mountain in which the elevation of the surface changes 
about 1,000 feet or more by a slope rapid enough to be 
plainly perceptible to the eye. If the slope be perceptible, 
but the change of elevation be much less than 1,000 feet, 
the region is called a hill. A relatively high point from 
which the surface slopes perceptibly in all directions, is 
called a peak. A long but very narrow region from which 
the surface slopes downward mainly in two opposite direc- 
tions, is called a ridge of mountain or hill. By far the 
greater number of mountains in the world occur in the form 
of ridges, or of ranges or chains ; that is, a succession of 
closely adjacent ridges, whose lengths lie along the same 
general course. A relatively high region, composed of 
two or more roughly parallel mountain chains, separated 
by elevated land, constitutes a mountain system. 

The dividing line between mountains and hills, based upon alti- 
tude alone, is purely arbitrary. Eminences called mountains in 
flat regions, would be called hills in regions where much higher 
eminences occur. A better plan would be to confine the term 
"hill" to relatively low eminences, composed of rock arranged in 
nearly horizontal layers. 

Plateaus and Plains are extensive regions having a 
comparatively flat surface, or one whose general slope is so 
gradual as to be scarcely perceptible. Such regions are 
generally called plains in lowlands, and plateaus in high- 
lands ; but where the lowland rises imperceptibly into 



THE SURFACE OF THE LAND. 163 

highland, the apparently flat or gently undulating surface 
is called a plain in both regions. 

This is the case with the Great Plains east of the Rocky Mount- 
ains, which slope imperceptibly downward to the east from an eleva- 
tion of about 6,ooo feet. On the other hand, relatively high, flat 
regions of the lowlands, when separated by steep slopes from lower 
regions, are frequently called plateaus ; thus, the greater part of the 
Cumberland and Appalachian plateaus lies at a less elevation than 
2,000 feet above the sea. 

Valleys are usually understood to be long, V-shaped 
depressions, whose side slopes are very perceptibly steep, 
and whose bottoms have a much more gradual slope in the 
direction of the valley's length. Valleys occur in every 
region of the land, but are more numerous, deeper, and 
all their slopes are steeper in highland than in lowland 
regions, and among hills, mountains, and plateaus than 
on plains. Indeed, it is the great number of very deep 
and steep valleys which give to mountain regions their 
very rough and uneven contour. 

The term valley is frequently used in a broader sense to include 
all the relatively low region lying between contiguous regions of 
highland. Thus, most of the United States between the Rocky and 
Appalachian mountains is said to lie in the Mississippi Valley. In 
this case the general slope of the sides of the valley is impercepti- 
ble, and is broken by steeper minor slopes into mountains, hills, 
plateaus, plains, and smaller valleys. 

Steepness of Slopes. — All plainly perceptible slopes 
are generally supposed to be much steeper than they 
really are, while imperceptibly sloping surfaces of course 
seem level. The Great Plains east of the Rocky Mount- 
ains have an average slope of about seven feet to the 
mile. This is entirely imperceptible. Probably an incli- 
nation of between 200 and 30O feet to the mile is required 
before any slope can be detected in the absence of a level 
surface with which to compare it ; such a slope makes an 
angle of about 3 with the horizontal. The great majority 



164 



PHYSICAL GEOGRAPHY. 




Slopes. 275 ft. to the mile 



30° _ Z. 40° 

3000 ft. tothe'mfle 4500 ft. iothe mfh. 

Fig. 70. 



of steep slopes make an angle of less than 30 with the 
horizontal; that is, they rise at a rate of less than 3,000 
feet to the mile, while slopes of 40 (4,500 feet to the 
mile) occur only in naked rock, and except when very 
short are exceedingly rare. Actual vertical precipices are 
never very high, for not only does a slope, or talus, tend 
to form against the bottom of the cliff by the accumula- 
tion of fragments detached from the top by the weather 
(Fig. 71), but the enormous weight of the overlying strata 
would crush the rocks forming the bottom of a very high 
cliff, and cause them to "creep" outward, thus reducing 
the lower vertical part of the cliff to a steep slope. 







Fig. 71.— A Line of Cliffs, with Talus (Red Gate, Utah). 

Some of the steepest general slopes in the United States are shown 
in the diagrams opposite. In nature they are broken by minor irreg- 
ularities which render them for very short distances alternately steeper 
and flatter than represented, but the diagrams show the average 
or general slopes, and the height of these is seen to be in general 





Horizontal and Vertical scales the same. 

GREEN MOUNTAINS 

„ ,. Mt. Grey loch 

isN. W. 



MASSACHUSETTS. 



ALLEGHANY MOUNTAINS, 

Alleghany Mts. 



3 MILES 

WEST VIRGINIA. 



Appalachian 

Plateau 



ROCKY MOUNTAINS, 
w - Pikes Peak 

-)5000- 



COLORADO 




-6000-FTt-ABOVE-SEA- 



WASATCH MOUNTAINS, 
Timpanogos Peak 




MOUNT SHASTA 



CALIFORNIA 
Mt. Shasta 




GRAND CANON (near toroweap), ARIZONA. 

Colorado _„„„ _, Kanab 

-Plateau 




-1000-FT-r-ABOVEJSEA 



Profiles of Steep Slopes. 



(165) 



1 66 PHYSICAL GEOGRAPHY. 

much less than the length. One general law is well illustrated by 
these diagrams : almost all slopes of the land surface gradually become 
flatter as they are descended. The reason for this will be explained 
later (page 220). The steepest long slope shown in the diagrams 

is that of the sides of the Grand 

"\ Canon of the Colorado River, where 

u. in the surface falls about -3,000 feet in 

^^^ I - less than half a mile. This Canon is 

3. often described as having nearly per- 

3) § Q pendicular sides. This is not the 

^\ J « j case, as is shown in the enlarged dia- 

"4 j - 1 M gram (Fig. 72), which shows the pro- 

4 o | g file of one side of the inner gorge of 

"-^s^^ 7 '' this canon near Toroweap, where the 

slope, though not quite so long, is 

about as steep as at any other point. (P. 223.) 

Highlands and Lowlands of North America. — North 
America contains two great mountain systems: the Appa- 
lachian system in the east, and the Cordillera system in 
the west. Each system is composed of numerous ranges 
or ridges, roughly parallel with each other and with the 
respective coasts of the grand division. The Cordillera is 
much the larger system in every way. It is bordered by 
two great chains, the Rocky Mountains on the east, and 
the Cascade Mountains, the Sierra Nevada, and the Sierra 
Madre of Mexico on the west. Between these are many 
isolated ranges. In each of these chains are many peaks 
between 12,000 and 15,000 feet high, while near Mt. St. 
Elias in the north are peaks over 19,000 feet high. In 
the south Orizaba rises over 18,000 feet above sea- 
level. These chains and the numerous shorter ranges 
and ridges between them rise from a rough plateau which 
maintains a general elevation of over 6,000 feet east of 
the Wasatch Mountains of Utah, but of less than 5,000 
feet to the west of it. This relatively low portion of the 
plateau extending west and south-west from Great Salt 
Lake to the Sierra Nevada, is called the Great Basin. 




MOUNTAINS, 

HIGHLANDS, AND 
LOWLANDS OF 

NORTH & SOUTH 
AMERICA. 

U'M Lowlands (below 2000 ft) 
tHi Moderate highlands. 
^M High highlands. 
— — Mountain ranges. 
Drawn in Lambert's Projection. 

(167} " 



1 68 



PHYSICAL GEOGRAPHY. 



Toward the northern and southern extremities of the sys- 
tem active volcanoes occur, while in the western part of 
the central portion numerous volcanic cones and other 
evidences of recent volcanic action are found. 

The Appalachian system throughout the southern 
portion of its extent consists of many sharp, parallel 
ranges or ridges rising from lowland elevations of less than 
1,000 feet to a general elevation of between 2,000 and 
3,000 feet above the sea. The general elevation of the 



5-MILES-W; 




MILES MILES 

ON PARALLEL OF 40 LATITUDE 
(Heights exaggerated 100 times) 

Height 



N.E. 




.=p^=T' " MILES M'l-ES 

'^p'' ON MERIDIAN OF 90° W. LONGITUDE 

Fig- 73.— Two Sections Across North America. 



eastern range is greater than that of the western ranges. 
Its highest peaks are Black Dome (6,700 feet) in North 
Carolina, and Mt. Washington (6, 200 feet) in New Hamp- 
shire. From the summit of the western range the Appa- 
lachian "plateau," with an elevation of a little less than 
2,000 feet, slopes westward, merging imperceptibly into 
the Mississippi Valley. 

The portion of the Appalachian system lying north of the St. 
Lawrence River is called the Laurentide Mountains, and is very 
different from the southern portion of the system. It is virtually a 
low plateau having an elevation of about 2,000 feet, from which rise 
occasional more or less isolated peaks or short ridges, which are 
worn to a smooth and rounded outline ; the height of these peaks is 



THE SURFACE OF THE LAND. 1 69 

generally less than 3,000 feet above the sea, though the highest is 
thought to exceed this elevation. Both the Appalachian and Lauren- 
tide mountains contain many evidences of very ancient, but none of 
recent volcanic action. 

The Lowlands of North America lie chiefly between 
the two mountain systems, and extend from the Gulf of 
Mexico to Hudson Bay and the Arctic Ocean. Although 
broken by short slopes into valleys, local undulations, 
and hills, which, in the case of the Ozarks of Missouri, 
and the Wichitas of Indian Territory, are dignified by 
the name "mountains," still the general slope is entirely 
imperceptible, and rises from both north and south to a 
maximum elevation of about 1,800 feet in the Height of 
Land north of the Great Lakes. 

South America contains three mountain systems: the 
Cordillera of the Andes, extending along the whole west 
coast, the Brazilian system in the east, and the Pacaraima 
system in the north. 

The Cordillera of the Andes, though much narrower, 
is almost twice as high as the Cordillera system of North 
America. It consists in general of two main chains 
roughly parallel with each other and with the west coast, 
from which the surface rises by a very steep slope, the 
crest of the westerly chain lying in some places not more 
than 65 miles from the sea-shore, and bearing many peaks 
between 16, ooo and 20,000 feet in elevation. Aconcagua, 
the highest peak, rises almost 24,000 feet above the sea. 
The eastern chain is not quite so high as the western; 
but near the central portion, where both chains are high- 
est, it has several peaks of 20,000 feet and more. This 
system contains throughout its length many active or re- 
cently extinct volcanoes. 

Between the central portion of the chains is a very high, broken 
plateau, whose elevation ranges from 12,000 to 14,000 feet. In the 
north, the eastern chain bears gradually off to the east parallel with 
P. G.-io. 



170 



PHYSICAL GEOGRAPHY. 




(Heights exaggerated 100 times) 



B ■azilian Highland a 



MILES Ml 

JUST SOUTH OF EQUATOR 



2-MILES-&; — 1 


■gg 






1 N * 


1 N^S* 

1 3.5 


PARAGUAY 


VALLEY 


4£<i 


A M 


^ Z O N 


V A L L^E 


Y 


ORINOCO VAL. 


SO 


5«. 


10 


00 


15 


00 


2000 25 


00 


3000 


3500 



BUENOS AYRES TO MOUTH OF ORINOCO 
Fig. 74. -Two Sections across South America. 

the Caribbean sea-coast, while the western chain, decreasing in alti- 
tude, bears to the north-west to form the highland connection, 
through the Isthmus of Panama, with the Cordillera system of North 
America. 

The Brazilian and Pacaraima systems are much 
lower and less continuous than the Andes. The highest 
peaks are about 9,000 and 8,000 feet respectively, though 
peaks higher than 6,000 feet are extremely rare. The 
Brazilian system is composed of three, and the Pacaraima 
of two, chains roughly parallel with each other and with 
the nearest coast. The Brazilian system rises rather 
abruptly from a low and narrow coast plain, but the Paca- 
raima system and the landward side of the Brazilian 
system have their base on a highland which attains its 
elevation of about 2,000 feet by an entirely imperceptible 
ascent. The eastern highlands of South America, like 
those of North America, contain no vestiges of recent vol- 
canic activity, but indications of ancient volcanism are 
found. 

The Lowlands of the greater part of South America 
are exceedingly flat. The Amazon River, where it leaves 
the Andes, 2,000 miles from its mouth, is only 500 feet 
above the sea. In eastern Bolivia, where the western 
highland approaches the Brazilian highland most closely, 



THE SURFACE OF THE LAND. 171 

the lowland between them is scarcely 1,000 feet above 
the sea, while west of the Pacaraima system the lowland 
has but half this elevation. 

Euro-Asia, the largest grand division of the land, and 
the most irregular in shape, has the most extensive, the 
highest, and the most irregular mountain system, as well 
as the most extensive lowlands. 

Mountains and Plateaus. — The highland region ex- 
tends along the entire southern portion of the grand divi- 
sion from Portugal to Bering Strait, a distance of 10,000 
miles. Though cut down to sea-level in one place — the 
outlet to Black Sea — this highland region, with its various 
plateaus and mountain ranges, may be regarded as a 
single vast mountain system. The general width and 
height of the system increase from either extremity 
toward the center, where the highland region is 2,500 
miles broad, from north-west to south-east. The plateaus 
at either extremity of the system have an elevation of 
about 2,500 feet, which gradually increases to about 5,000 
feet near the central region, where the surface abruptly 
rises to form the extensive Pamir-Thibet plateau, at an 
elevation of between 12,000 and 15,000 feet. This high 
plateau has a length of about 2,000 miles and an average 
width of 450 miles, an area equal to that of the United 
States east of the Mississippi. The broad plateaus of 
Asia are generally lower in the center than at the mar- 
gins. Thus, the central parts of the Persian, East Tur- 
kistan, and Mongolian plateaus are 1,200, 2,200, and 3,000 
feet respectively, while at the base of the surrounding 
mountains their elevation is about 5,000 feet. 

Though when distant mountain ' ranges of Euro-Asia are com- 
pared, they vary greatly in the direction of their trend, yet when the 
system is viewed as a whole the various chains are seen to be 
roughly parallel with the axis of the highland region, with the south- 



172 



PHYSICAL GEOGRAPHY. 



ern or eastern coast of the grand division, and with adjacent chains. 
Where the highlands are broad, the mountains generally rise from 
their northern and southern margins, inclosing the plateaus between. 
Several active volcanoes occur in this great mountain system, and 
many signs of recent volcanic action are found throughout its 
extent from Spain to Kamchatka. The gradual increase in the 
altitude of the mountains toward the central region of the high- 
lands is as follows : 



Spain, LoP'i 

Switzerland, 
Caucasia and Persia, 
Pamir and Thibet, 
Thian Shan, 
Khin Gan, 

Stanovoi Mountains, 



5 W. to o°. 
8° E. to I5°E. 
42° E. to 50° E. 
70° E. to 90 E. 
8o° E. 
105 E. 
117 E. 
135° E. 



Peaks, 1 1,000 to 12,000 feet. 
" 13,000 to 16,000 " 
" 17,000 to 19,000 " 
" 22,000 to 29,000 " 
" 21,000 feet. 
11,500 " 
9,000 " 
4,000 " 



Three wholly or partially detached regions of highland lie south 
or east of the main mass, being separated from it, however, by low- 
lands of small extent in comparison with the great lowland region 
to the north. These highlands constitute the plateaus of Arabia, 
India, and the coast region north of Corea. They have an average 
altitude of less than 2,500 feet, and are bordered by mountains of 
very moderate elevation. A mountain range, partially submerged, 
and roughly parallel with the east coast, forms the peninsula of 
Kamchatka and the chain of islands of which Japan, Formosa, the 
Philippines, and New Guinea are the largest. Through Borneo and 
Celebes this chain is connected with another more continuous chain, 
which, diverging from the east end of the Thibet plateau, forms the 
Malay peninsula and the islands of Sumatra, Java, and Timor. 
Most of these islands contain peaks of from 9,000 to 13,000 feet 
high, and very many of these are active volcanoes. 

Lowlands. — The northern part of the grand division is 
a vast region of continuous lowland, having an extreme 
length of 10,000 miles, and a greatest width, in the longi- 
tude of the Caspian Sea, of about 2,500 miles. The 
lowest part of this region is covered by the Caspian Sea to 
a depth of 3,600 feet, but the surface of the sea is still 85 
feet below sea-level. From this depression the surface 



= ^-n c g o g 

rS •3?3 5g"C" 30 $ * o 

» l g<5.S.«os o > > S 

o p »"s.S H = > o S > 

-- _ r= «■ „> m o ^~o>2 




(173) 



174 



PHYSICAL GEOGRAPHY. 



S.E. 














e> 




3.N.W. 




Kohrud Mis. 


Elbrooz fats. |^ 






J 


8 


3 n 


B 


A 1 


,l c 


i s p i a n L _ 




a 


5_ 


* la 


1 -S°- 


P E^R S| 1 


A l\ 


Sea = -5> R 


u 


s 


s 


l<? A 


oa 


A ;§" 




/^...^^Tr^r 


— ^ 


|s^ 




--3— 


-^y\ 


m=: 


1 












y== 



500 MILES 



ARABIAN SEA NORTHWEST TO- LOFODEN IS. 
(All ucrtiual distances exaggerated 700 times') 




BAY OF BENGAL TO KARA SEA 
Fig- 75- — Two Sections across Euro-Asia. 

rises to the north by imperceptible slopes to an elevation 
of about 1,000 feet, whence it imperceptibly descends to 
the north coast. These plains are called steppes in the 
south, and tundras in the north. This vast region rises in 
two places only, and then by almost imperceptible slopes, 
above the limit of lowlands (2,000 feet): (1) In the ex- 
treme north-west, to form the Scandinavian plateau, which 
falls off abruptly toward the sea from a general elevation 
of 2,000 to 4,000 feet, and bears some points over 8,000 
feet high ; and (2) between Europe and Asia to form the 
low Ural Mountains, whose highest points are about 5,500 
feet. In the extreme east the lowlands are broken by several 
ranges of hills or low mountains putting out from the great 
system to the south. In both of these isolated highlands, 
vestiges of very ancient volcanic action are found. 

Surface of Africa. — The main highland region extends 
along the eastern coast from the outlet of the Red Sea to 
the Cape of Good Hope. This highland increases in 
general width from north to south, and almost completely 
covers the southern portion of the grand division. 

Three long tongues of highland extend to the north- 
west from the main mass until they gradually merge into 
the lowland. These tongues are separated from each 




(175; 



I76 PHYSICAL GEOGRAPHY. 

other by two broad lowland valleys extending southward 
from the great lowland region which occupies northern 
Africa. One of these valleys is occupied by the Nile 
River, while the other contains the upper course of the 
Kongo, Lake Chad and its principal tributaries, and the 
upper course of the Niger. Minor elevations in the 
second valley have caused the lower course of the Kongo 
and Niger to bend at right angles with their upper course, 
and to cut narrow channels to the sea through the south- 
western highland tongue. A small, detached mass of 
highland in the extreme north-west extends parallel with 
the Mediterranean and Atlantic coast, and forms a contin- 
uation of the Italian region of elevation, which curves 
sharply back to westward through Sicily and the shallow 
extension from that island to Tunis (see page 152). 

It will be observed that the main highland mass lies roughly 
parallel with the east coast, while the three tongues are roughly- 
parallel with each other and with the north-east and south-west 
coasts of the grand division. Several active volcanoes and many 
indications of recent volcanic action are found in the eastern high- 
land, while in the west but one active volcano occurs on the main- 
land, though evidences of ancient volcanism are numerous. 

Heights. — The greatest heights of the grand division 
rise as more or less continuous mountain ranges from the 
margins of the highlands, and thus inclose a plateau whose 
general elevation is something less than 5,000 feet. The 
greatest heights occur along the eastern margin, peaks ris- 
ing to nearly 15,000 feet in Abyssinia, and to 20,000 
feet near the equator, where Kilimanjaro, the highest 
point in Africa, occurs. The peaks near the Zambezi 
sink to 7,000 or 8,000 feet, but rise again to 9,000 or 
10,000 feet in the extreme south. Along the west coast 
the mountains are lower, their peaks rising from 5,000 
feet near the Orange to 13,700 feet in the volcanic Cam- 
eroon Mountains at the head of the Gulf of Guinea, and 



THE SURFACE OF THE LAND. 



177 



Central 



Highland 



Libyan 




THROUGH THE SAHARA DESERT ( 26° N. LAT. ) 
Kilimanjaro ^ 




i y ON PARALLEL OF 10 S. LATITUDE 

l_Z^' {Heights exaggerated 100 times) " v <r_5 

Fig. 76. — Two Sections across Africa. 

thence sinking through the highland west of the Niger 
River to about 4,500 feet above sea-level. 

The southern range of the Atlas Mountains in the north-west is 
nearly twice as high as the northern, and contains one peak of 
nearly 12,000 feet. The plateau between these ranges has an 
average elevation of about 3,000 feet. The highest peaks of the 
central tongue of highlands are about 8,000 feet, and of the north- 
eastern tongue about 7,000 feet. The main highland region is 
divided by mountain ranges into several basin-shaped plateaus. The 
highest, 6,000 to 7,000 feet, extends, with a width of 200 miles, from 
Abyssinia to the equator, a distance of 1,200 miles. South of this, 
the plateaus maintain an average height of about 3,500 feet, having 
a central elevation of about 2,500 feet, and a marginal altitude of 
about 4,500 feet. 

The Sahara lowland has elevations of about 1,200 
feet between the central tongue and the north-western 
highland, and on either side of the Lake Chad depression. 
Lake Chad is about 800 feet above the sea, while in the 
north, limited areas between Tunis and the Nile are de- 
pressed to about the level of the sea, being in some 
places as much as 167 feet below it. 

Australia forms the extremity of a relatively small 
branch from the convex side of the main continental 
plateau ; that is, Australia is connected with Asia by a 
system of narrow, complex wrinkles in the earth's crust, 



178 PHYSICAL GEOGRAPHY. 

whose crests in some localities do not rise quite to sea- 
level, though throughout the greater part of the distance 
between the continents the crests of the wrinkles pro- 
trude above the sea to form the long and generally narrow 
islands of the Malay Archipelago. 

The axis of this whole region of elevations from 
Asia to Tasmania forms a sharply reversed curve. In the 
north the curve is concave toward the north-east, while in 
the south it is concave toward the south-west. The map 
(page 175) indicates that the law of the main continental 
plateau is equally true of this branch ; namely, the land is 
more continuous and the plateau is higher on the convex 
than on the concave side of its axis ; thus, in the north 
the Sunda Islands form a nearly continuous rim of land 
on the convex west side. They are separated from each 
other by narrow straits of relatively shallow water, and rise 
in many peaks to an elevation of 12,000 feet. They con- 
tain more active volcanoes than any other region of equal 
extent in the world. Gilolo and the Philippines, on the 
concave side, are much more discontinuous, are separated 
by very deep water, and contain peaks of only 5,000 to 
8,500 feet. Volcanism, though active on this side, is less 
so than in the Sunda Islands. The southern part of the 
axis just reverses the high and low sides of the plateau. 
From New Guinea to Tasmania the eastern and convex 
side extends as an almost continuous region of highland, 
with peaks of 13,000 feet in New Guinea, and of over 
7,000 feet in the Australian Alps; while on the concave 
side, the short west coast of Australia is the only land, and 
is quite low, rising in but two isolated instances to as much 
as 3,000 feet above the sea. In New Guinea and New 
Zealand there are active volcanoes; in Australia there are 
none, but signs of comparatively recent activity occur in 
the east, and of very ancient action in the west. ' 



THE SURFACE OF THE LAND. 1 79 

Transverse wrinkles cross the northern part of the axis to form 
the great islands of Borneo and Celebes, while across central Aus- 
tralia a broad transverse wrinkle or swell carries the surface gradu- 
ally up to a general elevation of nearly 2,000 feet,- and in places of 
over 4,000 feet above the sea. The south-east slope of Australia 
drops off rapidly to form the sea bottom at a depth of 15,000 feet, 
which then rises gradually to the crest of the sharply curved New 
Zealand wrinkle parallel with the Australian coast, and reaching in 
the volcanic peaks of those islands an altitude of 12,000 feet above 
the sea. 



* Indian 



Mo Donnell Mrs! I Aus. ATpT 



Pacific 



Ocean 



r^X, 



Ocean 



1 ■' MILES MILES \ '--- - -| 

j -/ ON TROPIC OF CAPRICORN \ _,. ' 2 

_W ,. / {Heights exaggerated 100 times) £_ 

Fig. 77. — Section Across Australia. 

Summary. — Thus, all the grand divisions contain mount- 
ain ranges, which are most numerous and -highest on the 
side bordered by the nearly continuous highland rim ; that 
is, on the west side of the New World, but the east and 
south-east sides of the Old World. Therefore, the great 
lowlands of the world border the Atlantic Ocean, from 
which they are separated either by no mountains or by 
comparatively isolated ranges of moderate elevation. Ad- 
jacent mountain ranges in all grand divisions are as a rule 
roughly parallel with each other and with the general trend 
of the nearest side of the continental plateau. Evidences 
of volcanic action are found in nearly all mountain regions. 
Present volcanic action or evidences of relatively recent 
action are most common toward that side of the grand 
division which constitutes part of the highland or convex 
margin of the continental plateau ; while among the 
isolated highlands on the concave side of the continental 
plateau evidences of very ancient volcanic activity are 
most common. 



CHAPTER XIII. 

THE STRUCTURE OF THE LAND. 

For a thousand years in thy sight are but as yesterday when it is fast. — 
Psalm xc: 4. 

Elements. — The earth has been examined to a depth 
entirely insignificant in comparison with its diameter, but 
so far as it has been examined, it appears to be composed 
mainly of twelve elements, or simple substances. These 
elements compose -j^-ths of the earth's crust, and are: 
oxygen, silicon, aluminium, calcium, magnesium, potas- 
sium, sodium, carbon, hydrogen, sulphur, chlorine, and iron. 
The remaining -j-o-g-th of the earth's crust is composed of 
about sixty other elements. Among these rare elements 
are all the useful metals, excepting iron and aluminium. 

Minerals. — With few exceptions, the twelve abundant 
elements do not occur in a free state, but in chemical 
combinations with each other and with the other elements. 
The stony substances resulting from such combinations are 
called minerals. The most abundant and common min- 
erals are silica and its compounds. Next in abundance 
are the carbonates of lime and magnesia. The oxides of 
iron are almost as common, but not so abundant. 

Rocks. — Most rocks are mixtures of two or more kinds 
of minerals, the particles of each being often visible to 
the naked eye. Thus, the granites are essentially mixt- 
ures of feldspar, quartz, and mica; ordinary "trap" rocks, 
or lava, of feldspar and pyroxene ; sandstones consist 

mainly of particles of silica; limestones, of carbonate of lime; 

(180) 



THE STRUCTURE OF THE LAND. l8l 

and shales and slates, of silicates of alumina, the principal 
substance in clay. These grains are usually joined to- 
gether by a cement of some mineral, which differs more 
or less from the mineral particles. Lime, which forms 
the essential principle of most artificial mortars and 
cements, is found in very many rocks as the natural 
cement binding together the particles, while peroxide of 
iron and silica serve this purpose in many other instances. 
The various colors of rocks, clays, and earths are very 
generally due to minute quantities of iron distributed 
through them in various combinations. 

Soil. — All rocks disintegrate — that is, crumble to 
pieces — more or less rapidly when exposed to the weather ; 
the process is therefore called weathering. In consequence, 
the surface of the solid rock over most of the earth is 
covered with a varying thickness of its own loosened and 
detached mineral particles. This loosened mass constitutes 
the soil. The surface soil is constantly being removed 
particle by particle — chiefly by the wash of rains to the 
nearest stream, but sometimes by winds as dust — while 
the rock beneath constantly breaks into soil. The disinte- 
gration and removal of the rock together constitute the 
process of erosion. 

The chief agents in the disintegration of rocks by weathering are : 
solution, change of temperature, the beating of rain, gravity, veg- 
etation, and winds. 

(i) Solution. — Some rocks are completely dissolved by percolat- 
ing water, but the majority are slowly broken up into particles by 
the solution of the cement which binds together the more insoluble 
grains. 

(2) Change of Temperature. — The hardest rocks are cracked ^by 
the expansions and contractions accompanying sudden changes of 
temperature. The crevices thus begun are opened by repeated ex- 
pansions of water freezing within them. 

(3) The beating of rain overcomes the cohesion of the softer 
rocks, and assists solution and frost by detaching loosened particles. 



182 



PHYSICAL GEOGRAPHY. 




Fig. 78.— Layers of Stratified Rock. 



(4) Gravity. — When the base of a cliff is greatly eroded, the 
upper part breaks off and falls from its own weight. 

(5) Plants often pry apart rocks by the growth of their roots, but 
their chief aid to disintegration is by increasing the solvent power 
of percolating water. 

Classes of Rocks. — The solid rocks beneath the soil 
may be divided according to their structure or arrange- 
ment into two classes : stratified and unstratified. 



THE STRUCTURE OF THE LAND. 1 83 

Stratified Rocks include sandstones, limestones, and 
shales, and occur as a general rule nearest the surface of 
the earth. They compose the surface rocks of about nine 
tenths of the land. In some places their thickness is 
known to be very slight; in other places there is good 
reason to think that they extend to a depth of at least 
ten miles. The average thickness of stratified rocks over 
the land is probably between two and three miles. 

Stratified Rock is characterized (1) by its arrange- 
ment in sheet-like layers, or strata, (Fig. 78), which vary 
from the thinness of a sheet of paper to a thickness of 
many feet. (2) By a more or less thorough assortment of 
the different minerals, and their collection into different 
strata ; thus, in each stratum some one kind of mineral is 
usually greatly in excess of all other kinds. (3) By con- 
taining imprinted on their surfaces, or imbedded in their 
mass, traces of animals or plants which must have existed 
before the rock was formed. These traces of former life 
are called fossils. (4) By being largely composed of 
mineral particles whose irregular shapes indicate that they 
are merely fragments from some older rock mass. 

These peculiarities can only be explained by sup- 
posing that these rocks resulted from the gradual com- 
pacting and hardening of beds of sand or mud, which had 
been deposited in water as sediment. Such beds of sedi- 
ment are now forming in every body of quiet water, and 
are found of every degree of hardness and compactness, 
from that of the softest mud to that of the hardest rock. 
It is certain that at least most of the stratified rocks are 
of such origin. They are therefore called sedimentary, 
aqueous, or fragmental rocks. 

Formation of Sedimentary Rocks. — The loose soil on the surface 
of the land affords directly or indirectly the greater part of the min- 
eral particles which compose the sediment collecting on sea and 



184 PHYSICAL GEOGRAPHY. 

lake bottoms; hence, the disintegration of the rocks into soil is the 
first step in the formation of future rocks. Particles of sand, clay, 
and carbonate of lime predominate in most soils, but are all mixed 
together in endless confusion. Through the force of running water 
and of gravity the particles are assorted and eventually transported 
to some lake or to the sea, where they are deposited in more or less 
distinct beds. The material of pure sand (silica or quartz), owing to 
its hard and durable nature, disintegrates very slowly, and thus, 
speaking generally, forms the largest and heaviest fragments in 
sediment. The heaviest particles, of course, sink soonest ; hence, 
sand predominates in the deposit nearest the shore, which gradually 
consolidates into sandstones of different varieties. The material of 
clay is derived from the chemical decomposition of feldspar, and is 
in very fine particles ; hence, it does not sink so soon as the heavier 
particles of silica, but is carried to greater distances from the shore, 
where it predominates in the sediments and consolidates into various 
kinds of shale. Fragments of carbonate of lime are also carried 
down from the land, and are deposited according to their size and 
weight ; but as this mineral is more or less soluble, these fragments 
grow smaller the longer they are in the water. Much carbonate of 
lime, therefore, reaches the sea in solution, and is generally distrib- 
uted by ocean currents as a chemical ingredient of the water. From 
this ingredient aquatic plants and animals derive material for their 
shells and skeletons. Upon the death of the organisms, these sink 
toward the bottom as sediment. Life is so abundant in many parts 
of the sea that where the water is shallow these shell fragments ac- 
cumulate on the bottom faster than the water can dissolve them. 
When this occurs in regions where but little sand or clay sediments 
are accumulating, beds of mud of nearly pure carbonate of lime 
may be formed, similar to, but not exactly like, the organic deep sea 
oozes. The consolidation of such beds produced by far the greater 
part of our limestones. 

Unstratified Rocks underlie the stratified rocks and 
extend indefinitely into the interior of the earth. In some 
places the unstratified break through the stratified rocks, 
thus forming the surface rocks over about one tenth of the 
land. Unstratified rocks include the granites ; the finer 
grained rocks, called trap ?vcks, as trachyte, basalt, and 
obsidian ; and the still finer grained modern lavas. 



THE STRUCTURE OF THE LAND. 1 85 

Peculiarities. — The texture of unstratified rocks is 
peculiar in being either smooth and glassy, or, if granular, 
the grains or particles have more or less distinctly the reg- 
ular shape and structure of the crystals peculiar to the 
mineral of which they are composed. Now, melted rock 
in cooling assumes this same glassy or crystalline texture, 
and this, with the absence of fossils, suggests that heat 
was an essential agent in the formation of the unstratified 
rocks. Hence, they are often called igneous (fire) rocks. 

An igneous origin is also indicated by the manner in 
which unstratified rocks occur; namely, (1) as structure- 
less and irregular shaped masses, forming the core of 
some mountain chains; (2) as lava "dikes," filling great 
fissures across the beds of the stratified rocks, as though 
lava had been forced into these fissures from below when 
in a melted state ; and (3) as lavas, overlying stratified 
rocks, as though they had welled up through a volcanic 
vent or a fissure, and spread out over the surrounding 
surface before cooling. 

Not infrequently dikes and sheets of unstratified rock are wholly 
composed of regular columns arranged side by side and closely fit- 
ting together. Now, the contraction of a bed of fine mud, as it 
dries in the sun, frequently causes cracks to traverse its surface in all 
directions, subdividing that surface into more or less regular shaped 
areas, which, as the cracks often penetrate deeply into the mud bed, 
are really but the ends of a series of columns similar to those found 
in the rock. Hence, the columnar structure of rocks is thought to 
have resulted from a similar cause, namely, the contraction of the 
rock mass in cooling. 

Metamorphic Rocks. — Certain rocks, including slate, 
quartzite, marble, etc., possess more or less distinctly 
the stratified arrangement of the sedimentary rocks, 
with the crystalline texture of the igneous rocks. They 
are often of aqueous origin, but their original frag- 
mental texture has subsequently been changed more or less 

P. G.— 11. 



1 86 PHYSICAL GEOGRAPHY. 

perfectly to a crystalline texture ; they are therefore called 
changed, or metamorphic, rocks. 

The causes of this change or metamorphism are 
heat, moisture, and pressure, while the change is greatly 
facilitated by the presence of certain common minerals. 
In order that the mineral molecules in a fragmental rock 
may assume a crystalline arrangement, a certain freedom 
of movement, as exists in pasty substances, is necessary. 
Under ordinary conditions it requires a temperature of 
nearly 3,ooo° to melt or liquefy rocks, but when thus 
melted, all stratification would disappear. When, how- 
ever, rocks under great pressure are heated in the pres- 
ence of even a very minute quantity of water so placed 
that it can not expand into steam, and especially if certain 
minerals are in solution in the water, the rock begins at 
temperatures of only between 200° and 300 to pass into a 
state which seems to allow of crystallization and of new 
chemical combinations, but does not destroy the stratifi- 
cation. 

As the earth is penetrated, it becomes warmer at a rate which at 
a depth of 2>% miles would produce a temperature 300 above that 
at the surface. Now; in some places the stratified rocks are more 
than 3^ miles thick ; it is therefore evident that the temperature of 
the bottom strata in such places must be at least 300 higher than 
when these bottom strata formed the surface of the deposit. As the 
weight of the sediment above would exert great pressure, and as all 
rocks contain more or less water percolating through them, which 
water, too, is frequently impregnated with just the necessary minerals, 
it is more than probable that even at this depth the conditions are 
generally favorable for the partial crystallization or metamorphism 
of the lower strata. As the thickness of the deposit increased by 
the continued accumulation of sediment on top, the heat, pressure, 
and metamorphism in the lower strata would increase, while the 
strata above would begin to crystallize, until finally the heat in the 
lower strata might become so great as to destroy all trace of stratifi- 
cation, and convert the rocks into true unstratified or igneous rocks. 
Since metamorphism takes place only at great depths, and since 



THE STRUCTURE OF THE LAND. 1 87 

metamorphic rocks could not have retained the stratified structure 
had they ever been rendered soft enough to admit of their being 
forced through the overlying rocks to the earth's surface, it follows 
that crystalline rock, having a stratified structure, occurs at the 
earth's surface only when it has been denuded, or laid bare, by the 
gradual disintegration and removal of a great thickness of rock 
which once covered it. The occurrence of metamorphic rock at the 
surface of the earth is thus of itself an indication of extensive erosion, 
or denudation (see Chapter XVIII). 

The Primitive Rock, or that which first formed over 
the earth's surface by the gradual cooling of the molten 
globe, must have resembled the present igneous rock in 
containing no fossils, in being unstratified, and in having 
a crystalline or glassy texture. But nearly all crystals 
contain numberless microscopic cavities. In slags and 
lavas, which are known to have solidified slowly, like the 
primitive rock, from a melted state, these cavities are 
filled with the mineral of the crystals, but in a glassy con- 
dition. In crystals, however, which have been produced 
artificially by a process similar to that which resulted in 
metamorphism, many of the cavities contain nothing but 
water. Now, in most of the unstratified or igneous 
rocks, many of these crystal-cavities contain water, which 
indicates that they are aqueous rocks which have under- 
gone complete metamorphism. In fact, it is probable 
that none of the primitive rock now remains on the earth's 
surface in its original position and condition, but that dur- 
ing the ages which have elapsed since its formation this 
rock afforded the mineral particles to soil and sediment, 
which eventually completely covered its surface, and 
through progressive changes became successively the strat- 
ified-fragmental, the metamorphic, and the unstratified- 
crystalline rocks which compose the present surface, and 
which are now supplying through similar processes these 
same particles to other similar cycles of change. 



1 88 PHYSICAL GEOGRAPHY. 

The Land was once Submerged. — Since most, if not 
all, of the rocks of the land are thus but more or less com- 
pletely changed sediments, it follows that the present land 
must at some time have been entirely under water in 
order that the sediment might accumulate. All of the 
land could not have been under water at one time, how- 
ever, for it has been seen that the bulk of the sediment 
comes from the disintegration of an adjacent land surface. 
Hence, we conclude that adjacent areas of the land have 
alternately been depressed below and then elevated above 
the surface of the sea — perhaps many times — the area 
above the sea supplying the material which was deposited 
on the adjacent depressed area, which, when subsequently 
elevated, supplied material for the sediment collecting in 
surrounding depressed regions. 

This makes it evident that no stratum of sedimentary rock is 
continuous over very wide regions, but that each, considered as a 
whole, is a great cake, whose thickness decreases gradually in all 
directions from the region which, at the time of its deposit, was 
nearest to the source of supply, and hence received the most sedi- 
ment. 

Disturbed and Faulted Strata. — That such movements 
of elevation and depression are possible, is proved by the 
fact that in many localities the coast regions of the land 
are observed to be very gradually rising above or sinking 
beneath the adjacent water surface, while such movements 
are proved to have taken place in regions far from the 
present coasts by the position of the strata. Sediment 
will not rest at all on a steeply sloping bottom, and its 
deposition tends to lessen gentle slopes. It is therefore 
certain that most of the sedimentary strata were originally 
nearly or quite horizontal. As actually found, however, 
the strata are seldom exactly level ; they every- where show 
more or less distinctly traces of tilting or curvature, as 



THE STRUCTURE OF THE LAND. 



189 



though they had been thrown from their original hori- 
zontal position into a series of great, wave-like undulations. 
The surface at the crest and trough of such a rock wave 
was of course elevated and depressed respectively when 
the movement took place. In some places the waves are 
short and high, in which case the strata composing their 
sides slope, or dip, at a steep angle ; in other cases, the 
waves are so long and flat that the dip of the strata is im- 
perceptible. Frequently the strata are found to be broken 




Fig. 79. 



across, and the strata on one side of the break to have 
been upheaved above or depressed below the correspond- 
ing strata on the other side. This is called a fault. 

It seldom happens that the strata can be actually seen continu- 
ously from the crest to the trough of a wave ; they generally dip 
down out of sight into the earth in one direction, while the top of the 
wave has been carried away piecemeal by erosion, leaving only the 
ragged edges of the strata to compose the earth's surface. The 
shape of the wave, however (shown by the dotted lines in the dia- 
gram, Fig. 79), is indicated by the various dips of the adjacent strata. 
Erosion has in the same manner quite generally carried away the 
upheaved side of faults, so that their position is indicated by a sud- 
den change in the character of the rock rather than by a sudden 



190 



PHYSICAL GEOGRAPHY. 



change in elevation in the earth's surface. Many circumstances, 
such as the enormous erosion, prove that these faults and flexures 
of the strata were not produced by great single movements, but that 
each is the aggregate result of thousands of slight movements of a 
few inches or a few feet, occurring at irregular but very long intervals 
of time. These slight movements are still taking place in all parts 
of the world, and are, as we shall see later, the cause of earth- 
quakes. There is probably no locality in which these movements 
are always in the same direction, either upward or downward ; but, 
in general, the convex or Pacific side of the continental plateau 
seems to be slowly rising, while, with certain local exceptions, the 
concave or Atlantic side seems to be gradually sinking. 




Fig. 80. — Unconformable Strata. 



Unconformity of the Strata. — In some places a series 
of strata A (Fig. 80), having a certain dip, rest directly 
upon the eroded surface of another series, having either 
the same dip (B), or an entirely different one (C). The 
two series are then said to be iinconformable . 

Such a position indicates several movements of the earth's crust; 
thus, (1) an upward movement of the sediments B or C to bring 
them above the water that they might be exposed to the weather and 
eroded ; (2) a downward movement to allow the deposit of the sedi- 
mentary rock A beneath the water, and (3) an upward movement to 
convert this deposit into the present land. 

Relative Age of Rocks. — When the relative position 
of rocks has not been greatly disturbed by subsequent 
movement, it indicates the order of formation or relative 
age of the rocks, the highest being the youngest and the 
lowest the oldest. But this method involves the direct 
comparison of the position of rocks, and therefore applies 



THE STRUCTURE OF THE LAND. 



I 9 I 



only to rocks in the immediate neighborhood of each 
other. To determine the relative age of the rocks com- 
posing widely separated regions — on different continents, 
for instance, or on opposite sides of the same continent — 
some other method has to be used, since it is never possi- 
ble to trace the strata from one region directly to the 
other over the intervening distance. The only known 
method for identifying the relative periods in geological 
time, at which rocks in widely separated regions were 
formed, is by a comparison of their fossils. 




Fig. 81. — Rock Fragments, showing Embedded Fossils. 



Fossils. — The harder parts of land or aquatic animals 
and plants are sometimes buried in adjacent accumulations 
of sediment and preserved for long ages. When at last 
they decay, a hollow mold having their shape or outline 
is left in the hardened deposit, and is gradually filled up 
solid by the precipitation of some mineral in solution in 
the water percolating through the. deposit. Thus a frag- 
mentary record of the forms of life which existed at the 
lime each layer of sediment was deposited is preserved 
within the rock stratum, either as the organic remains 
itself, its empty mold, or as a stone cast (Fig. 81) filling 
up this mold, until metamorphism effaces, more or less 
completely, both the lines of stratification and the fossil 
contents of the rocks. 



I92 PHYSICAL GEOGRAPHY. 

Sometimes the precipitation from the percolating water replaces 
the organism particle for particle as it decays, thus preserving in 
stone all the delicate internal structure of the organism. Such 
fossils are called petrifactions. In other cases, only a portion of the 
substances liberated by the decay of the organism escapes, and the 
residue recombines into a new substance which may or may not 
retain the outline of the organism. Coal, asphalt, petroleum, and 
"natural gas " are the new substances which, under different circum- 
stances, result from this process (page 369). 

Identification of Relative Age of Strata by Fos- 
sils. — Careful examination of the fossils in thick series of 
stratified rocks, whose relative age is indicated by the rel- 
ative position of the strata, reveals that the fossils at the 
bottom are not quite the same as those at the top of the 
series. As the series is ascended, different kinds of fossils 
gradually disappear, while other kinds gradually make 
their appearance, and are in turn replaced by newer forms. 
It thus appears that each stratum contains a few kinds of 
fossils not found in any other strata. These peculiar 
fossils, though not always the most numerous, are called 
the type fossils of that stratum. When similar type fossils 
are found in widely separated regions, they always succeed 
each other in the same general order; that is, the older 
fossils in one region are also the older in other regions. It 
has thus been established that the gradual changes in the 
forms of life in the past have taken place in the same 
general order all over the world, and that similarity of 
type fossils serves to identify corresponding strata in 
widely separated regions, and to afford a clue to the relative 
ages of rocks. 

It is probable that these changes in life forms took place more 
rapidly in some regions than in others ; hence, the occurrence of 
similar type fossils in widely separated regions does not necessarily 
indicate that these organisms lived at exactly the same time, but that 
they lived when the changes of life forms had reached correspond- 
ing periods in the two regions. 



THE STRUCTURE OF THE LAND. 



193 



Classification of Rocks. — The rocks which expose 
their edges at some point or other of the earth's surface 
are classified according to their relative age. The thou- 
sands of strata are divided into five great groups, each of 
which marks an era of time: (1) Azoic (lifeless) or 
Eozoic (dawn of life), the oldest, in which all the strata 
yet found are so completely metamorphosed that the fossils 
are either effaced entirely or rendered unrecognizable ; (2) 
Paleozoic (ancient life), or Primary, in which most of the 
strata have been metamorphosed, but some retain their 
fossils as the most ancient recognizable forms of life ; (3) 
Mesozoic (middle life), or Secondary, in which metamor- 
phosed strata are frequent ; (4) Cainozoic (recent life), or 
Tertiary, in which metamorphism is quite exceptional ; 
and (5) Post Tertiary, or Quaternary, which includes fossils 
of the present forms of life, and in which no metamor- 
phosed strata are found. The strata composing these 
groups are subdivided into 
systems, each marking a 
period of time ; these into 
series, marking epochs of 
time ; and these again into 
stages, marking ages of 
time ; while the stages 
are composed of beds or 
individual strata. 

Geological Time. — If 
any of the numerous 
changes in the past which 
are indicated by the study 
of the rocks be compared 
with the rate at which 
similar changes are taking 
place Jn the present, the 



Eras of Time. 


Periods of Time. 


r^ , I Recent 
Quaternary. \ pleistocene . 


Tertiary 


Pliocene. 


or 


Miocene. 
- Eocene. 


Cainozoic. 


Secondary 


" Cretacecus. 


or 


Jurassic. 


Mesozoic. 


. Triassic. 




" Permian. 


Primary 


Carboniferous. 
Devonian. 


or 
Paleozoic. 


Silurian. 
Cambrian. 




- Archaean. 


Azoic 

or 

Eozoic. 


Not subdivided, because 
as all stratification and 
fossils have been de- 
stroyed by metamorpho- 
sis, nothing remains to 
determine the relative 
ages of different parts of 
k the group. 



194 PHYSICAL GEOGRAPHY. 

conclusion is irresistible that geological time must be very, 
very long. If only the small thickness of sediment de- 
posited in one year by even the muddiest water be 
compared with the very great average thickness of the 
sedimentary rocks, one becomes convinced that many 
thousands or even millions of years have been required 
for these rocks to accumulate. There is no way to deter- 
mine the exact length of geological time. Some circum- 
stances indicate that at least 100,000,000 years must 
have elapsed since the oldest known sedimentary rocks 
were deposited ; other circumstances indicate that it could 
not have been more than 3,000,000 years, but neither of 
these estimates is accurate — the time may be greater, or it 
may be less. All that can safely be affirmed is that the 
fragmentary record of the earth's history which the sedi- 
mentary rocks afford, covers a very long period of time. 

It is perhaps impossible for the human intellect to grasp the lapse 
of time comprehended in the expression "one million years." By 
a great effort of memory, an old man may appreciate the length of 
not much more than half a century ; and yet if half a century be 
represented by a distance of three inches, a million years would be 
represented by one mile. 



CHAPTER XIV. 

THE WATER OF THE LAND SPRINGS. 

He sendeth the springs into the valleys, which run among the hills. They give 
drink to every beast of the field. — Psalm civ: io, ii. 

The vapor of the atmosphere, through its condensa- 
tion into rain, snow, dew, etc., supplies all the water 
encountered on the surface of the land. This may be 
classified according to its manner of occurrence, as springs, 
streams, glaciers, and lakes. 

Permeability of Rocks. — All rocks can absorb more 
or less water. Clay and fine grained, compact rocks, 
though they may contain water, do not allow it to escape 
readily, and are therefore said to be impermeable. A 
layer of soil, sand, or coarse grained, loosely cohering, 
or much fissured rock, on the contrary, allows water to 
pass through it copiously, or is said to be permeable. 

Invisible cavities between the mineral particles, and visible fissures 
make up from one sixtieth to one half the bulk of most rocks. 
Water is absorbed or forced into these cavities by its own weight 
(gravity) and by capillary attraction (adhesion). When the cavities 
are very minute, capillary attraction is stronger than gravity, and 
holds the water fast in the cavities, making the rock impermeable 
though it contains water. When the cavities are large, gravity is 
stronger than capillary attraction, and the water sinks through the 
cavities and escapes from the rock below. 

Surface Springs and Wells. — When the surface rocks 
are permeable, a large part of the rain or snow water 
sinks through them until it reaches and saturates an im- 
permeable stratum. Being unable to escape through this 

(195) 



196 



PHYSICAL GEOGRAPHY. 



RELATIVELY HIGH LAND 




VALLEY 



■ Impermeable . Stratum 7: 

Fig. 82. 

stratum, the water accumulates in and saturates the over- 
lying rocks to a height (s, Fig. 82) where its own pres- 
sure forces it to move slowly along the depressions in the 
surface of the impermeable stratum. If this stratum is at 
such a slight depth that its edges crop out on the sides or 
in the bottom of the adjacent valleys, the water issues 
along this line of outcrop as surface springs. During wet 
weather the water collects in the rocks above the imper- 
meable stratum faster than it escapes at the springs ; the 
upper limit of saturation (s) therefore rises, its elevation 
being approximately marked by the surface of the water 
in wells. During dry weather the continued flow of the 
springs causes the limit of saturation to fall. If it should 
fall below the line s all the springs would dry up, although 
a well (wi) penetrating below this line would still supply 
water, while a shallower well (iv2) would be dry. When 
the limit of saturation is very high (as si), the increased 
pressure frequently forces the water to the surface at unus- 
ually high levels, forming wet weather or intermittent 
springs, which flow only until the excessive pressure is re- 
lieved by the lowering of the limit of saturation. 

Deep-seated Springs and Artesian Wells. — When 
inclined strata outcrop at the earth's surface, and are 
arranged in such a manner that permeable strata are in- 
closed between impermeable strata, the rain or snow water 
which sinks into the permeable strata at their outcrop is 
confined in these strata by the impermeable beds above 
and below. To whatever depths the permeable beds may 



SPRINGS. 



197 



descend, this water necessarily follows, and may in this 
way travel underground for many miles and reach depths 
of thousands of feet, until stopped by the gradual thin- 
ning out of the stratum, by its sudden ending in a close 
fault, or by the high temperature of the earth at extreme 
depths. When the descent of the water is stopped from 
any cause, the strata gradually become saturated up to 
the lowest level of their outcrop. The water in the sat- 
urated strata toward the lower end of the incline is of 
course pressed upon by the weight of the water toward 
the upper end. This pressure is often great enough to 



Level 0/ Out, crop 



Outcrop 




Fig. 83. 



force the water to clear a passage for itself through some 
small fissure or other channel in the overlying imperme- 
able beds, and, rising in this channel, to gush forth as a 
deep seated spring or natural fountain in a region where 
the surface is at a lower level than the surface region where 
the permeable beds receive their supply of water. An arti- 
ficial channel of this kind, produced by drilling a hole 
through the impermeable strata, constitutes an artesian 
well, so called from its early use in Artois, France. 

Theoretically, the water will rise through the well to the same 
height as the outcrop of the permeable strata ; but the obstruction 
offered to the flow of water by the permeable rock and the leakage 
through the confining strata considerably reduces the height to 
which the water will rise. Where these two factors are very small, 
the water has been found to rise to the surface when the surface at 
the well is about as many feet below the surface at the outcrop, as the 



I98 PHYSICAL GEOGRAPHY. 

two localities are distant from each other in miles. If this difference 
in height is much greater, the water may rise from the well into the 
air as a jet or fountain. Artesian wells are now very common — 
almost every city in America and Europe containing one or more. 
They are especially valuable in regions having a dry climate, as in 
the western portion of the Union and the desert portion of Algeria. 
These wells vary greatly in depth in different localities : one in 
Berlin is 4,172 feet deep; one in St. Louis, Mo., is 3,843 feet; in 
Budapest, Hungary, over 3,000 feet ; in Cincinnati, O., 2,408 feet; 
in Louisville, Ky., 2,086 feet. 

Use of Springs. — By absorbing and temporarily re- 
taining a large part of the rainfall, the permeable rocks 
prevent devastating floods which otherwise would accom- 
pany every heavy rain, while by the gradual surrender 
of the absorbed water in springs the supply of fresh water 
at the earth's surface is maintained through ordinary 
seasons of drought. 

Devastating floods sometimes occur, it is true, but are almost in- 
variably due to the rapid melting of snow by warm rains at a time 
when the underlying soil is either completely saturated or is rendered 
temporarily impermeable by frost. In ordinary summer droughts, 
such streams as the Ohio and upper Mississippi, which are not sup- 
plied at that season by melting snows, contain only spring water. 
Should the drought continue long enough, the springs would exhaust 
the underground supply, and such streams would dry up. 

The temperature of the water in springs is nearly con- 
stant throughout the year. It frequently seems warm in 
winter and cool in summer, but it is really the temperature 
of the air and surface rock which varies — the spring water 
seeming warm or cool in comparison. The temperature of 
different springs, however, varies greatly. It is rarely less 
than 40 Fahrenheit, but may range upward to the boiling 
point. When the temperature of the water is much 
higher than the mean temperature of the surface rocks in 
the vicinity, the spring is called a warm or thermal spring. 
Springs slightly warmer than the surface rocks are com- 
mon, and springs much warmer are by no means rare. 




Artesian Well at Prairie du Chien, Wis. 



(i99) 



200 



PHYSICAL GEOGRAPHY. 



In the region of volcanic rocks between the Rocky Mountains 
and Sierra Nevada, perceptibly warm springs are the rule. East of 
the Rocky Mountains they are more exceptional, but are found in 
nearly every state ; — a very few are named below : 







Temp. 


Mean 


g 


Flow 


Name. 


Locality. 


of 


Temp. 


u 


Gal. Per 






Spring 


Surf. 


W 


Hour. 


Lebanon Springs, 


Columbia Co., N. Y. 


75° 


46° 


29° 


30,000 


Warm 


Bath Co., Va. 


98 


46 


52 


360,000 


Sweet 


Monroe Co., W. Va. 


79 


46 


33 


48,000 


Warm " 


Meriwether Co., Ga. 


90 


60 


3° 


84,000 


Hot 


Garland Co., Ark. 


157 


62 


95 


20,100 


Palmyra 


Jefferson Co., Wis. 


72 


46 


26 






Blankenships " 


Texas Co., Mo. 


75 


57 


18 


2,000 



Spring water derives its temperature from the rocks through which 
it percolates, and the rocks at a very slight depth cease to be 
affected by the daily and seasonal variations of surface temperature. 
Since the rocks in non-volcanic regions become warmer at an 
average rate of i° for each 50 feet of increased depth, and since the 
water percolates very slowly, it has time to acquire the rock tempera- 
ture during its downward passage. It follows more or less open 
channels in its journey upward or outward to springs, and flowing 
more rapidly does not lose all of its acquired heat. In non-volcanic 
districts the excess of temperature of spring water affords a very 
rough approximation of the depth from which it has come. The 
Arkansas Hot Springs have an excess of 95 , and must come from 
a depth of nearly a mile. The water of deep artesian wells is 
almost always perceptibly warm : that at St. Louis has a tempera- 
ture of 105 , and that at Louisville of 76^°, the mean temperature 
of the air at those places being 55 and 57 respectively. 

Spring Water. — In percolating through the rocks, the 
water is constantly dissolving and carrying along with it 
soluble minerals. In addition to this, it is constantly caus- 
ing chemical changes, by which new and soluble substances 
may be made from insoluble minerals. Frequently these 
new substances are gases, with which the water is charged 
when it arrives at the surface. The most common gas 



SPRINGS. 20 1 

thus produced is carbonic acid, which, escaping in minute 
bubbles, causes the usual "sparkle" of spring water. The 
gas sulphuretted hydrogen causes the disagreeable odor 
of most "sulphur" springs. Thus, strictly speaking, all 
springs are mineral springs, but only those are usually so 
called in which the mineral or gaseous contents impart a 
perceptible taste or peculiar medicinal quality to the water. 

The minerals which most commonly occur in spring water are : 

Carbonate of lime ) . . , 

y making temporary hard water, 

magnesia J . 

" " iron, " chalybeate water. 

Sulphate of lime ( pybsum), ) x . , 

r ,. . ,^f ,_,, y permanent hard water, 

magnesia (Epsom salt) J 

Chloride of sodium (common salt), " saline water. 

Nitrate of potassium (saltpeter), 

Sulphate of sodium (Glauber salt), 

Bicarbonate of sodium (common soda), 



Sulphate of alumina with the \ ( t < 
Sulphate potassium or sodium J 
Silica, making silicious water. 



making alkaline water. 



It is the relatively large or small quantity of lime or magnesia 
contained in the water which renders it hard or soft. Soap produces 
lather in soft water ; in hard water it does not. If these minerals 
are present as carbonates, they may be removed from solution by 
boiling ; if they are sulphates, the water is permanently hard. 

Caverns, Sink-holes, and Spring Lakes. — Caverns, 
or subterranean tunnels and chambers, are formed by the 
prolonged solution and abstraction of mineral matter by 
percolating water. In some limestone districts, owing to 
the solubility of this rock, such caverns are often many 
miles in extent. Mammoth Cave, in Kentucky, and the 
Luray Caverns of Virginia are noted instances. By the 
falling in of the roofs of caverns, or by the solution of the 
rock along the vertical joints that serve as channels for de- 
scending rain-water, sink-holes are formed — such as occur 

P. G.— 12. 



202 PHYSICAL GEOGRAPHY. 

in the blue grass region of Kentucky. The surface drain- 
age, creeks, and even large streams may disappear in sink- 
holes directly underground, where they greatly hasten the 
work of cave formation. The falling of the roof of a 
cavern, by obstructing an underground stream, might 
cause the water to rise through the debris and form a lake 
in the sink-hole. The enormous springs of Florida, as 
Silver Spring and Orange Spring, into which steamboats 
can ascend, as well as many large spring basins in other 
limestone regions, were probably formed in this manner. 

Since mineral matter in solution does not impair the clearness of 
spring water, the amount abstracted from the rocks is seldom appre- 
ciated. Average spring water contains more than xo§oo tns °f i ts 
weight of dissolved mineral matter. The springs of the United 
States east of the Mississippi are almost innumerable, but the dis- 
charge of water from only 900 of them has been measured : these 
collectively bring to the surface each year a quantity of the under- 
ground rock equal to a mass 10 feet square and two miles long. 

Deposits of Springs and Percolating Waters. — 

The power of water to dissolve most minerals increases 
with its temperature and the amount of gases it contains. 
Percolating water at great depths, therefore, generally dis- 
solves more mineral matter than it can hold in solution 
when it reaches the surface, where it cools, and, being re- 
lieved of pressure, much of its carbonic acid gas escapes 
to the atmosphere or is absorbed by aquatic plants or 
mosses. Hence, deep-seated springs are usually sur- 
rounded by a deposit of the minerals with which the 
water is impregnated. Sometimes this deposit may even 
form large hills ; sometimes it forms a mound around the 
spring, over the sides of which the water falls, while the 
spray, evaporating from surrounding objects, leaves them 
also incrusted with a mineral deposit. Percolating water 
evaporating on the sides and roof of limestone caverns, 
leaves the walls incrusted with carbonate of lime in beau- 



SPRINGS. 



203 




Fig. 84. — Scenes in Mammoth Cave, Ky. 

tiful masses of crystals. Water slowly evaporating as it 
drips from the roof of caverns to the floor beneath leaves 
a deposit on both places, which gradually grows down- 
ward from the roof as a stalactite, and upward from the 
floor as a stalagmite, until these meet and form one con- 
tinuous column of stone. 

The deposit of calcareous springs, or travertine, may be white, 
or, if iron is also present in the water, it may be yellow, brown, red- 
dish, or beautifully striped. Chalybeate or iron spring deposits vary 
from bright yellow to brown. Sulphur is frequently deposited by 
springs impregnated with sulphuretted hydrogen, and white siliceous 
sinter, by hot springs in volcanic districts. 



204 



PHYSICAL GEOGRAPHY. 



Land-slips. — Absorbed water lessens the cohesion of 
most rocks ; it renders impermeable clay more or less 
plastic and slippery, and tends to soften many permeable 
limestones and sandstones. When saturated, rocks at 
some depth below sloping surfaces may thus allow the 
overlying rocks to slide downward under the pressure of 
their own weight, especially when that is increased by the 
weight of an unusually large quantity of percolating water 
during wet weather. Such movements are called land- 
slips (page 261). 




Fig. 85. 



Fig. 86. 



Small land-slips (Fig. 85) in which the soil and subsoil to a depth 
of a few feet slip a short distance downward, are common on all hill- 
sides, especially where the subsoil is underlaid by strata of clay, 
as is the case in the vicinity of Cincinnati. Large land-slips are 
generally confined to hilly or mountainous regions, where the strata 
are inclined at nearly the same angle as the surface. In such local- 
ities serious land-slips are not uncommon. Some stratum, as AB 
(Fig. 86), has its cohesion weakened by moisture until it is not 
able to support the weight of the overlying mass C, which suddenly 
starts downward, carrying with it the forests, houses, and every thing 
on its surface. The mass overwhelms whatever it meets, and may 
form a natural embankment across the valley at D. By obstructing 
the flow of the drainage, such an embankment may cause the for- 
mation of a more or less permanent lake on its upper side. Many 
mountain lakes have thus been formed by land-slips. 



CHAPTER XV. 



STREAMS. 



Then the channels of -water appeared, and the foundations of the world -were 
laid bare. — Psalm xviii: 15. 

Streams are bodies of water flowing in definite chan- 
nels from a higher to a lower level over the earth's sur- 
face. The water in streams is the excess of the rain-fall 
on the land over evaporation. Streams are called rills, 
brooks, creeks, and rivers as they increase in relative size. 

Sources and Mouth. — The beginning of a stream, at 
the higher level, is called its source. The source of 
a stream is generally a spring, which, it has been seen, is 
but the re-appearance of absorbed rain-fall ; but the source 
may be a mass of melting ice or snow, a lake, a swamp 
or marsh, or simply the water of a shower that flows over 
the surface after the soil is completely saturated. The 
place where a stream joins or flows into a larger stream, 
a lake, or the sea, is called its mouth. 

General Law of Streams. — Water, when free to 
move under gravity, always flows to the lowest attainable 
level and by the steepest path it can find. Therefore, 
streams always occupy lines of depression, or valleys. Hence, 
streams generally increase in size as they advance in con- 
sequence of the constant addition of water from the sides 
of the valley. This water collects in the depressions in 
the valley sides, down which it flows as minor streams or 
tributaries to the main stream in the bottom of the valley. 

(205) 



206 PHYSICAL GEOGRAPHY. 

Thus, the Ohio and Arkansas rivers drain parts of the valley 
sides, and are tributaries to the great Mississippi ; the Wabash, 
Miami, and Licking rivers perform the same office in the smaller 
Ohio valley, and are tributaries to that river; the Whitewater and 
Mad rivers are similarly tributaries to the Great Miami ; and so on 
down to the smallest screams, whose tributaries are mere threads 
of water, hidden, perhaps, under the grass or fallen leaves. 

Stream Systems and Drainage Basins. — A stream, 
and all the lesser streams that contribute water to it, con- 
stitute collectively a stream system. The whole surface 
of the land whose inclination is such that it contributes 
water in time of wet weather to any stream of a system 
is called the drainage basin, or simply the basin of the 
main stream of that system. 

Water-shed. — The boundary line of a drainage basin is 
called the water-shed, the water parting, or simply the 
divide, between that and adjacent basins. The location 
of water-sheds is exactly the reverse of that of streams; 
they always occupy lines of elevation. The crest of 
every sharp ridge forms a water-shed, but the top of an 
imperceptible swell in an apparently level prairie is also a 
true water-shed. Hence, a water-shed may be defined as. 
the irregular line of relatively high land formed by the 
meeting of opposing slopes, whether the slopes are long 
or short, fiat or steep. 

The chart (Fig. 87) shows the main water-shed, the drainage basin, 
and the principal streams constituting the system of the Mississippi 
River. In the west and east respectively the water-shed generally 
follows the lofty crests of different ridges of the R.ocky and Appa- 
lachian mountain systems, but in each locality it sometimes crosses 
from one ridge to another, following the highest part of the inter- 
vening valley. In crossing from one mountain system to the other, 
the water-shed follows the line which is continuously the highest 
across the intervening low country. Near the head of Lake Michi- 
gan this great water-shed, which divides the drainage of nearly the 
whole grand division, lies in the apparently level prairies scarcely 



STREAMS. 



207 




Fig. 87. 

600 feet above the sea. Each stream of a great river system has a 
minor stream system, basin, and water-shed of its own. 

Oceanic and Inland Drainage Basins. — The almost 
continuous highland region that lies near the convex mar- 
gin of the continental plateau forms the main water-shed 
of the land. Thence, the surface descends by long and 
gentle slopes toward the Atlantic and its great arms, but 
by comparatively short and steep slopes toward the 
Pacific and Indian oceans. These slopes embrace the 
drainage, or JiydrograpJiic, basins of the respective oceans. 
There are areas in each grand division where the rain-fall 
is so slight in comparison with the evaporating power of 
the air that all the streams are entirely evaporated before 
they can traverse the region. These regions of deficient 



208 PHYSICAL GEOGRAPHY. 

rain-fall, or of low relative humidity, and the territory- 
draining into them are called inland basins, because they 
contribute no streams to any ocean. By far the largest 
area of the land lies on the Atlantic side of the main 
water-shed. Fully one half of the land on the globe con- 
tributes its drainage to the Atlantic, and only about one 
eighth to the Pacific and Indian ocean basins respectively. 
The inland basins collectively cover about one fourth of 
the land surface. 

The Discharge of Streams. — No stream discharges 
at its mouth all of the rain-fall which occurs in its basin. 
In traversing the basin, the streams are diminished in 
volume (i) by evaporation, (2) by subterranean channels 
leading into some other basin or to submarine outlets, and 
(3) by chemical change of the water into some other sub- 
stance, either in the soil, in plants, or in animals. The 
diminution by evaporation is vastly greater than that from 
all other causes. The proportion of rain-fall discharged 
varies greatly in different basins, depending on the intri- 
cate local conditions which occasion the disappearance of 
water, such as relative humidity of the air, permeability 
of the surface rocks, character of the region with respect 
to vegetation, etc. No great basin discharges into the sea 
much more than one half the rain-fall it receives. The 
Yukon, the Magdalena, and the Rhine discharge about 
one half; the Amazon and the Mississippi about one fifth ; 
and the Nile, whose lower course traverses a rainless 
region, but ^7-th of the rain-fall of their respective basins. 
The average discharge into the sea from all streams in the 
world is estimated to be only one fourth to one fifth of the 
rain-fall on the land. This small proportion, however, 
amounts to about 6, 500 cubic miles annually — a volume 
of water great enough to cover the whole United States, 
including Alaska, to a uniform depth of 9^ feet. 




^Atlantic | Arctic | Of the Atlantic Ocean (inc. Arctic) 26.400.000 Sq. Miles, 

11 11 Inland basins 11.500.000 " '/ 

11 11 Pacific Ocean 7.500.000 // u 

" a Indian Ocean 6.700.000 ,/ „ 

(209) 



2IO PHYSICAL GEOGRAPHY. 

Relative Size of Streams. — The true measure of the 
absolute size of a stream or stream system is the volume 
of running water it contains. This volume changes from 
day to day and from season to season, and depends upon so 
many factors, that its determination is practically impossi- 
ble. The relative or comparative size of great stream 
systems is approximately indicated by the mean annual 
volumes of rain-fall occurring in their respective basins, 
which depends simply upon the mean depth of the rain- 
fall and the area of the basin. The opposite table indi- 
cates graphically the relative sizes of the thirty-three great 
river systems determined by this method. 

It will be noticed that some systems, as the Amazon, Kongo, La 
Plata, and Nile, owe their prominence both to the heavy rain-fall and 
to the great area of their basins. In others, as the Mississippi and 
the Siberian rivers, less than the average rain-fall is compensated 
by the great extent of their basins ; while in still others, as the 
Orinoco, San Francisco, Irrawaddy, and especially the Magdalena, 
small basins are compensated by exceptionally heavy rain-fall. The 
discharge at the mouth of a system, and the length of its longest 
stream, are sometimes used as indications of its relative size, but a 
large system may lose most of its water in its lower course and dis- 
charge a relatively small quantity of water, while a long stream may 
be shallower and have fewer and shorter tributaries than a shorter 
stream ; thus, the Nile, though three times as long as the Ohio-Alle- 
ghany, discharges only two thirds as much water, while the Mis- 
sissippi-Missouri, the longest stream in the world, discharges but 
little more than one fourth as much water as the Amazon. 



The Longest Rivers 


in the World, 




., 4,192 miles. 




Yenisei . . 


2,950 miles 


. 4,018 




Amoor . . 


2,919 " 


3.156 " 




Kongo . . 


2,88i " 


• 3.°6i 




Mackenzie, . 


2,866 " 



Nile . . 
Yangtze-Kiano- 
Amazon . 

The Largest Mean Annual Discharges. 



Amazon . 528 cubic miles. 
Kongo . 419 " " 
La Plata . 180 " 



Mississippi, . 145 cubic miles. 
Yangtze-Kiang 125 " " 
Orinoco . . 122 " " 



6 Orinoco 

7 Niger 

8 Ganges-Brahma. 

9 Yangtze-Kiang 

10 St.Lawrence 

11 Yenisei 

12 Zambezi 

13 Obi 

14 Lena 

15 Amoor 

16 San Francisco 

17 Meliong 

18 Danube 

19 Irrawaddij 

20 Volga 

21 Yukon 

22 Murray 

23 Sasliatchewan 

24 Hoang Ho 

25 Magdalena 

26 Maqhenzie 

27 Rio Grande 

28 Indus 

29 Columbia 

30 Euphrates 

31 Colorado 

32 Orange 

33 Dnieper 


STREAM SYSTEM 

/ Amazon 

2 Kongo 

3 La Plata 

4 Nile 

5 Mississippi 


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212 



PHYSICAL GEOGRAPHY. 



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Fig. 88. 

Slope. — No stream has a regular or uniform slope from 
its source to its mouth. If any stream be examined at 
low water, it will be found to present a succession of 
short but relatively steep descents, alternating with longer 
reaches where the fall is much less. Now, the speed or 
velocity of a stream depends mainly upon the inclination 
of its surface. The water creeps slowly over the long, flat 
reaches, but makes the shorter and steeper descents with 
a rush, forming rapids or ripples where the descent is an 
incline, but cataracts or cascades if the descent is vertical. 
If these minor irregularities of slope be disregarded, and 
only the general inclination be observed — as in the profiles, 
(Fig. 88) — the slope, though it varies in different parts of 
the course, will as a general rule be found, like that of 
mountain sides, to decrease in steepness as it is descended. 
The average fall per mile in the lower course of five of the 
rivers indicated above is from 2 to 6 inches, while near 
their sources it varies from 4 -4, to 120 feet to the mile. 



STREAMS. 



213 




Speed of Streams. — The velocity of streams is gen- 
erally a little greater just below the surface, than at the sur- 
face or nearer the bottom ; and is greatest near the middle 
of the stream. A stream may be considered as composed of 
a number of layers of water roughly parallel with the cross 
section of its bed (Fig. 89). The advance of each layer is 
somewhat retarded by 
friction. The bottom 
layer a is pressed upon 
by the greatest weight 
of water, and moving 
upon the irregular bed 
rock, its retardation by friction is greatest and it moves 
slowest. The friction of each successive layer above a is 
less than that of the layer upon which it moves, since it is 
pressed upon by a less weight of water. Hence, if the line 
xy represents the surface of the stream, the layer d oc- 
cupying the central portion of the surface is retarded least 
by friction, and therefore flows fastest. If, however, the 
surface is raised to wv during a freshet, the layer/, being 
farther from the bottom, flows still faster. 

The average speed of an ordinary river current varies from less 
than xyi miles an hour at low water, to less than 6 miles at high 
water, while exceptionally rapid torrents probably never exceed a 
speed of 20 miles an hour. The Ohio River at Cincinnati, where 
its fall is 4 inches to the mile, has a mean surface speed of \y% 
miles an hour when the water is low (6 feet deep). When the 
water is high (54 feet deep) the average current is nearly 6 miles an 
hour ; that is, it is 6.35 miles in the channel and 5.85 miles per hour 
half-way toward either bank. The Mississippi River at Baton Rouge, 
where the fall is 3 inches per mile, has a mean speed at low water 
of i}i miles, and at high water of 4 miles an hour. 

Variations in Volume. — Owing to the intermittent 
supply of rain and snow water, many streams are subject 



214 PHYSICAL GEOGRAPHY. 

to great variations in volume. Rains, or the melting of 
snow, over a considerable portion of any drainage basin re- 
sult in a greater or less rise of its streams. A short, 
heavy rain-fall, or the rapid melting of snow, though 
yielding a comparatively small volume of water, may, on 
account of its suddenness, cause a greater rise at a given 
locality than a greater but more gradual increase of vol- 
ume. An exceptionally great or rapid increase in the 
volume of water in any basin may cause the streams to 
overflow their usual banks or channels, and spread over 
the adjacent lowlands, producing a flood, called a freshet 
in small streams. 

The same volume of water causes streams to rise to different 
heights at different points along their course, depending upon the 
variations in the widths of the valley and the slopes of the stream. 
Where the valley is narrow, the same volume of water causes the 
stream to rise higher than where the water can spread out over a 
greater width of valley. High water tends to make the surface slope 
of streams uniform by increasing the slope on level reaches and de- 
creasing it at rapids. Thus, the greatest known flood in the Ohio 
(1884) caused a rise of 46 feet above the " Falls " (rapids) at Louis- 
ville, but of 72 feet three miles below, at the foot of the rapids. 

These fluctuations of volume, since they depend en- 
tirely upon the weather in the respective drainage basins, 
are always irregular in amount, and more or less irregular 
in time of occurrence. Local showers, falling on compar- 
atively small areas, may be occasioned by many purely 
local and temporary conditions of the atmosphere, and 
therefore occur at irregular intervals ; hence, small streams, 
whose basins lie more or less completely within the small 
areas of these local rains, fluctuate irregularly. 

The volume of water which flows at once over the 
surface from these showers, though ample to swell the 
smaller streams, is seldom sufficient to have a very per- 
ceptible effect upon the main streams of a great river 



STREAMS. 215 

system, whose basin embraces a very extensive area. 
Enough water to cause a marked rise in such streams is 
only supplied by wide-spread or long-continued rains or 
by the melting of extensive snow-fields. Such effects are 
caused chiefly by the varying amount of heat received 
from the sun at different seasons, and therefore depend 
largely upon the regular movement of the earth in its 
orbit. Hence, a certain corresponding regularity is noticed 
in the recurrence of fluctuations of all large streams. 

The Ganges and the Nile exhibit special regularity in time of re- 
currence of low and high water, but a greater or less degree of reg- 
ularity is exhibited in this respect by all large streams. The increas- 
ing heat of summer not only melts the snows on the lofty Himalayas, 
but causes the moist monsoon winds to blow from the ocean ; the 
resulting rains aid the snow-water in swelling the Ganges. An 
annual rise begins in May and continues till September. The rise 
is from 30 to 45 feet at Allahabad, and 7 to 10 feet at Calcutta. In 
the same manner the moist spring monsoons deposit excessive rain- 
fall on the highland of eastern Africa from Abyssinia to the equator. 
Thus, the headwaters of the Nile begin to rise annually about the~ 
1st of May, but it takes two months for the rise to reach Cairo, 
where, by about October 1st, the river has reached a height of be- 
tween 18 and 30 feet above its June level. Throughout the United 
States the rains are more uniformly distributed over the year, but 
the great Rocky Mountain tributaries to the Mississippi are gener- 
ally highest through June and July on account of the snow-water 
from their elevated sources. The snow about the less elevated 
sources of the Ohio, however, melts earlier in the season, and Feb- 
ruary is the month of flood in that river. In consequence of the 
different times of flood in its large tributaries, the minor fluctuations 
of the lower Mississippi are somewhat irregular, but it is always 
above its mean level from January 1st to August 1st, and below it 
during the rest of the year. The usual range between low and high 
water in the Missouri increases from about 6 feet at Fort Benton, 
Mont., to about 35 feet at its mouth ; in the Arkansas from 10 feet 
at Fort Gibson, Ind. Ter., to 45 feet at its mouth. The usual range 
of the Mississippi above Hannibal, Mo., is 14 to 20 feet, increasing 
to 50 feet from Cairo, 111., to Red River, and decreasing thence to 
nothing at its mouth. The range of the Ohio is about 50 feet. 



CHAPTER XVI. 



WORK OF STREAMS. 



The waters wear the stones ; thou wzshest away the things which grow out of 
the dust of the earth. — Job xiv : 19. 

Transportation — It has been said (page 18 1) that a 
process, called erosion, is constantly at work over the sur- 
face of the land, this process consisting of the disintegra- 
tion and removal of the land surface particle by particle. 
The weather is the principal cause of the general disinte- 
gration of the surface ; streams are the principal means of 
the removal of the disintegrated material. During its 
transport, each particle becomes a tool with which the 
stream powerfully and rapidly disintegrates and wears 
away its bed, even if the bed consists of the hardest rock. 
This method of disintegration, which takes place only in 
the beds of running streams, is called corrasion to distin- 
guish it from the more general disintegration of the whole 
surface of the land by the weather. Streams transport 
rocky material in three ways: (i) in solution, (2) in sus- 
pension, and (3) by rolling or pushing it forward along the 
stream bed. 

In solution. — While rocky material is in solution or 
dissolved in water, it is, strictly speaking, no longer rock, 
its molecides having been separated to form a constituent 
part of the water. When in this form the rocky material 
accompanies the water in all its movements and to any 
distance, until some change of temperature or pressure 

renders the water unable to hold it nil, when a portion of 

(216) 



WORK OF STREAMS. 217 

the dissolved matter is precipitated into its true rocky form 
again. 

In suspension. — The transportation of rock particles 
in suspension is entirely different, and is simply mechan- 
ical, depending upon the swiftness of the current and the 
size of the particles. Larger or smaller rock or soil par- 
ticles are constantly finding their way into streams by 
their own weight or by the force of the winds, but chiefly 
through the wash of successive rains. Once in the stream 
their more rapid journey begins. The rock particles, 
being heavier than water, have a tendency to sink, and 
would go straight to the bottom if the water were still ; 
but in its general advance over the irregularities of its 
bed, an intricate system of minor currents is set up in the 
body of the stream, which move upward and sideways, 
and occasion the "boiling" places, waves, and whirlpools 
always seen on the surface of rapid streams. These minor 
currents prevent the sinking of the finer rock particles, 
which are therefore carried along by the main current in 
suspension. Should the main current increase in velocity, 
the force of the minor currents increases, and larger par- 
ticles can be held in suspension ; should the speed of 
the main current become slower, the minor currents de- 
crease in force, and the larger particles in suspension sink 
to the bottom. It is the material in suspension that 
causes the muddiness or turbidity of stream water; and 
the general increase in its turbidity after rains is occa- 
sioned both by the large amount of fine soil particles 
washed in by the rain, and by the ability of the stream to 
carry along more and larger particles in suspension when 
its volume and velocity are increased by the shower. 

The capacity of running -water for material in 
suspension increases very rapidly as its speed increases; 
to double its speed would increase its capacity about 64 



21 8 PHYSICAL GEOGRAPHY. 

times. Hence, very much more material is transported 
by streams in times of flood, or high water, than when the 
water is low. 

If the material is fine enough to be held in suspension, the trans- 
porting capacity of water flowing at any speed is very great. If this 
capacity were reached, the stream would appear as a mass of very 
fine mud or " quicksand," just liquid enough to flow, and the water 
would form but one fourth of its weight ; that is, out of every five 
cubic feet of the liquid mass, three cubic feet would be solid par- 
ticles. It is very seldom that enough soil particles, small enough to 
be held in suspension, are washed into a stream at one time to fill it 
nearly to the limit of its transporting capacity ; but such mud- or 
sand-streams are occasionally encountered. 

Material pushed forward on the stream bed. — 
Innumerable particles and rock fragments, too large to be 
held in suspension, are yet small enough to be rolled for- 
ward along the bottom of streams with great force by the 
main current. It is to the attrition of such fragments 
and of those in suspension, that the wearing away or 
deepening of stream beds is chiefly due. The size of 
these fragments, the force with which they advance, and 
hence the amount of deepening of the stream bed, or 
corrasion, increase very rapidly with the speed of the main 
current. The deepening of any stream bed by corrasion 
of course increases the steepness of slope of its valley 
sides; hence, the speed and corrasive power of all its 
tributary streams are increased, and this increases the 
slope, speed, and corrasion of all streams flowing into 
these tributaries. Therefore, the deepening of any stream 
bed increases the amount of disintegration and transporta- 
tion — that is, of erosion — over its entire basin. 

Sedimentation. — Wherever the speed of a current is 
checked from any cause, the water is no longer able to 
hold the larger particles in suspension; they therefore 
settle to the bottom to be either rolled along or left be- 



WORK OF STREAMS. 2IQ 

hind as sediment, according to the force yet remaining in 
the main current. Should this current be further checked, 
the water becomes clearer as still smaller particles in sus- 
pension are deposited upon the bottom, until, upon the 
cessation of all currents, the water would become per- 
fectly clear as the smallest particles in suspension are 
gradually deposited. 

The amount of material transported varies greatly 
in different streams according to their slopes and the char- 
acter of their basins, and in any one stream it varies greatly 
with the stage of water. Many calculations on different 
rivers indicate that streams on the average transport about 
Y^fo^-ths* of their weight of mineral matter. That is 
to say, the rivers of the world, in the aggregate, trans- 
port each year from the land to the sea enough rocky 
material to make a sharp crested range of mountains 
1,000 feet high, a mile wide at the base, and 30 miles 
long. 

The quantity of material in suspension alone discharged annually 
by the Mississippi would make a range of hills 500 feet high, half a 
mile wide at base, and over a mile long. The Ganges discharges 
annually about the same amount of matter in suspension, while the 
suspended matter which the little Rhone discharges annually into 
the Mediterranean would make a pyramid a mile square at the base 
and 230 feet high. 

Formation of Valleys. — By thus removing the par- 
ticles disintegrated by atmospheric agencies, and by the 
attrition of these particles upon the stream bed, the 
streams themselves, during the long ages of the past, 
have hollowed out and formed the valleys in which they 
flow, and the same processes are to-day modifying the 



* In suspension, .000558 — T. M. Reade, Am. Jour. Science, 1885, page 21 

In solution, .000186 — J. Murray, Sc. Geog. Mag., 1887, page 76. 

Rolled along bottom, .000067 — Hump, and Abbott, Hyd. Miss. Riv., page 14 



Total, .000811 

P. G.-13. 



2 20 PHYSICAL GEOGRAPHY. 

shape of every foot of the land. The variety in the 
slopes and shapes of valleys results from the varying rate 
of corrasion and weathering in different regions as deter- 
mined by slope, climate, and hardness of the rocky 
material composing the earth's surface. 

The Curve of Erosion. — In spite of these causes of 
variation, the general slopes of different parts of the land, 
and of different valley bottoms in particular, have a rough 
resemblance in becoming flatter as they are descended. 
(See diagrams, pages 165 and 212.) This arrangement 
of slopes results from the invariable action of running 
water and the peculiar curve which it produces in the 
general slope may therefore be called the curve of corra- 
sion or erosion. 

The surface of the land is a succession of steep and gentle slopes. 
All parts of it are, constantly or intermittently, subjected to the action 
of running water ; — the beds of permanent streams, constantly ; and 
other parts of the land, during and immediately after rains. The 

stream or rain-water flowing 
swiftly down the steep slopes 
corrades more material than its 
slower current is able to trans- 
port over the flatter slope below ; 
hence, a deposit is formed 
against the bottom of the steep 
descents as at a, a, (Fig. 90). 
Subsequent action of the same 
kind causes, for similar reasons, further deposits at b and c. Thus, 
the profile of the slope gradually acquires the form of a succession 
of curves of erosion. But the deposits are formed of material cor- 
raded from the higher parts of the slope d, d, which are thus flat- 
tened as indicated by the dotted lines. When this part of the slope 
becomes as flat as the surface of the deposit at b, the checking of 
the current and the settling of sediment ceases, while corrasion 
begins to cut away the deposit at that point. Prolonged action of 
this kind gradually wears away all irregularities, and reduces the 
slope to a single curve of erosion (Fig. 91). The constant action 
and greater volume of water on stream beds has frequently reduced 




Fig. 90. 



WORK OF STREAMS. 



221 



their general profile to a single curve from mouth to source; but 

in many streams the reduction has not yet advanced so far, and 

two or more curves can be 

distinguished in the general / 

profile o'f the stream, as 

in the Colorado and the 

Nile (Fig. 88). The general 

surface of the land, acted 

upon by smaller volumes of 

water, and only at intervals, 

is reduced more slowly, and 




Fig. 91. 



its profile, except in its more general features, presents a long suc- 
cession of curves of erosion. The deposit of corraded material at 
the place where a current of water is checked by encountering a 
gentle slope occasions the familiar alluvial cones, or fan-shaped 

heaps, which invariably 
occur where swift mount- 
ain streams or the com- 
mon wet weather gullies 
of steep hillsides encounter 
the more gently sloping 
plain. 

The steepness of 
the sides of valleys 
depends upon the rel- 
ative rapidity of the 




Fig. 92.— Alluvial Cones in Utah. 



corrasion of the stream bed at the valley bottom, and the 
more general weathering of its sides by rain, frost, etc. 
Where corrasion is the more rapid, the valley is deepened 
faster than it is widened, and its sides are steep, giving it 
more or less the shape of a V. When weathering is the 
more rapid, the valley is widened faster than it is deep- 
ened, its sides become natter and lower, and its cross sec- 
tion is more basin-shaped. The general rapidity of 
weathering is usually about the same in the same material 
in all parts of small basins, but the corrasive power of the 
stream decreases rapidly as it advances down the succes- 
sively gentler slopes of its bed. Hence, as a general rule, 



2 22 PHYSICAL GEOGRAPHY. 

valleys are narrow and relatively deep in the upper course 
of streams, but gradually become wider and relatively 
shallower as the stream is descended, and in the lower 
course of large streams may become so wide and flat as 
to lose entirely all visible side slope and become practi- 
cally plains. Thus, the lower Mississippi valley, from 
Cairo to the Gulf, is a gently sloping plain varying in 
width from 20 to 70 miles. 

The rapidity of weathering on the valley sides is not 
always the same, however, throughout the same drainage 
basin ; it varies considerably with the amount of rain-fall. 
If part of the course of a stream traverses a region of 
either very heavy or very light rain-fall, the effect is im- 
pressed on the shape of its valley. Heavy rain-fall in- 
creases the rate of weathering of the valley sides and of 
the adjacent upland, and a proportionately wider and 
shallower valley is the result. Light rain-fall has the op- 
posite effect, and favors the formation of deep, narrow 
valleys, with steep side-slopes. Thus, the lower valley 
of the Nile has a very gentle slope, and corrasion is cor- 
respondingly slow ; but it lies in so dry a region that the 
rate of weathering is still slower, and a comparatively 
deep, narrow valley is produced. When the rain-fall is 
very slight and the slope of the streams is very great, a 
canon, or valley of exceptional narrowness in proportion to 
its depth, is formed. Noted examples of this are afforded 
by the Colorado, Virgin, and many other streams of the 
Rocky Mountain plateau region. 

The character of the material has an important influ- 
ence upon the general transverse shape of valleys. Thus, 
the streams which traverse the Great Plains, though they 
have a steep slope and traverse a region of scant rain-fall, 
produce valleys so wide and shallow as scarcely to merit 
the name valley. This is because the rock of the region 



WORK OF STREAMS. 



22 




Fig' 93- — Grand Canon of the Colorado, at Toroweap. 



weathers so rapidly that its surface is always covered with 
a great depth of sand and soil particles which slide into 
the streams from the sides as fast as other particles are re- 
moved from the stream bed by the current. Thus, the 
stream becomes overloaded with sediment, and maintains 
a deposit of sand upon its bed which it can not remove. 
The bed is thereby protected from corrasion ; hence, the 



2 24 PHYSICAL GEOGRAPHY. 

valleys are constantly getting wider without getting 
deeper. 

The rivers which flow from the Rocky Mountains across the 
Great Plains, like the Arkansas and the Platte, have unusually steep 
slopes, being about as steep as the Colorado. But, after leaving the 
mountains, they cut no canons or deep valleys, while the Colorado 
has cut profound ones. The difference in the two cases is due to 
the fact that the river troughs of the Great Plains are deeply buried 
in sand, the waters of the rivers being loaded to their utmost 
capacity, while the Colorado is able to transport more sediment than 
it receives. The rocks in its trough are to a great extent bare owing 
to the scouring action of the material in suspension, and the channel 
is continuously deepened. 

Variety in the Character of Material. — When hard 
strata alternate with soft ones, the sides of a valley form a 
series of steep and flat slopes, the steep slopes occurring 
in the hard strata. The rapid erosion of 
some soft strata frequently undermines 
the edges of a hard overlying stratum. 
A line of overhanging cliffs along the 
valley side is thus formed (Fig. 94). Frag- 
ments of the cliff often fall from their 
own unsupported weight, aided by the 
prying action of freezing water in the 
joints (page 13). If such fragments 
collect faster than erosion can remove 
them, they gradually form a talus, which may cover and 
for a time protect the softer strata from further erosion. 

Cataracts and Cascades. — If the main or a tributary 
stream in a valley whose sides contain lines of cliff be 
ascended until the stream bed reaches the foot of the 
cliff, a water-fall is encountered. It may be a cataract, 
called a cascade in small streams, or simply rapids, accord- 
ing to circumstances. If the strata are nearly horizontal, 
and the stream clear and large enough to reduce and carry 



Hard Strata 




WORK OF STREAMS. 225 

away the rock fragments about as fast as they fall, the 
overhanging form of the cliff is constantly maintained. 
The stream leaps over this as a cataract, leaving a space 
under the hard stratum and behind the falling water which 
is constantly filled with spray, and into which people can 
frequently enter from the sides. The occasional but con- 
tinued detachment and fall of fragments from the over- 
hanging cliff, causes a constant recession of the cataract 
up stream. 

The cataract of Niagara, midway between lakes Erie and Ontario, 
is about 165 feet high. Though by no means the highest, it is prob- 
ably the grandest cataract in the world on account of its great 
volume of water. Immediately above the fall the Niagara River is 
almost a mile wide. It flows over the brink with an average depth of 
about four feet, and a greatest depth of perhaps twenty feet. Enough 
water flows over every twenty -four hours to make a lake a mile 
square and 821 feet deep. Below the fall the river occupies a nar- 
row valley, or cafion, which gradually increases from 200 to 300 feet 
deep. The canon is so narrow that in it the river has only one eighth 
to one fourth of its former width. The stratum of hard stone that forms 
the brink of the cataract outcrops along the top of the cafion, form- 
ing a line of cliffs, beneath which a talus of fragments slopes steeply 
down to the water's edge. Seven miles below the falls these cliffs 
turn sharply away from the stream to right and left, and the river 
flows thence through a low, open country to Lake Ontario. The 
place where the cliffs turn away from the stream undoubtedly marks 
the original position of the cataract, while the seven miles of canon 
is the amount the fall has receded up stream from the constant un- 
dermining and breaking away of the hard stratum. Judging from the 
present rate of recession of the fall, about three feet a year, it has 
required 12,320 years for the excavation of the canon. It has prob- 
ably not required quite so long a time, however, for at present a 
large portion of the river — the American Fall — falls into the side of 
the cafion and is consequently engaged in increasing its width and 
not its length. The cafion is so narrow because the talus of hard 
rock fragments" from the cliffs on its side slopes forms a protecting 
layer over the soft strata beneath, and thus prevents to a great 
extent the undermining and downfall of the cliffs. 



2 26 PHYSICAL GEOGRAPHY. 

Rapids, or a series of very low falls, are generally 
formed instead of a single cataract where the strata have 
a very steep dip, or when the stream is muddy, or is not 
sufficiently powerful to prevent the formation of a talus 
under the edge of the horizontal strata; for in these 
cases the form of an overhanging cliff can not be main- 
tained, and the water simply rushes down a steep broken 
incline. Rapids are also formed by many other obstacles 
to the uniform descent of streams. 

Since within certain limits the corrading power of a stream in- 
creases with the amount of sediment it carries, muddy streams may 
wear away the hard stratum forming the brink of a water-fall faster 
than the weather erodes the soft strata beneath, and thus prevent 
the formation of an overhanging cliff and a cataract, the water sim- 
ply descending a steep incline as a rapid. Indeed, cataracts are 
almost invariably confined to streams carrying clear water, such as 
those issuing from lakes. Most of the conditions favoring the for- 
mation of great cataracts exist along the steep course of the Colo- 
rado River, and the fact that only rapids occur is believed to be due 
to the great corrading power which the large amount of sediment 
carried gives to its current. It is believed that the noble cataract at 
Niagara would be quickly reduced to a simple rapid were the water 
of the river very muddy instead of being very clear as it is. 

Immense age or permanence is often suggested by 
reference to hills or mountains, — such sayings as "old as 
the hills," "the everlasting mountains," etc., being com- 
mon. But the position of valleys, which are every-where 
and almost invariably the work of running water, proves 
that in very many instances the courses of the larger 
streams are older than the bordering hills and mount- 
ains. 

Instances of this are numerous in most regions, but are specially 
conspicuous in mountain districts. In the Appalachian region of 
New York, Pennsylvania, and Virginia the numerous parallel 
mountain ridges seem to have influenced but little the course of the 
larger streams, which flow directly through the various ridges one 
after another in a succession of deep, narrow gorges or water gaps, 



WORK OF STREAMS. 227 

such as the Highland gorge of the Hudson, the Delaware Water 
Gap, the Susquehanna Water Gap above Harrisburg, the Harpers 
Ferry gorge on the Potomac, etc. This indicates that the general 
course of these streams was established before the present mountains 
existed, and was maintained by the constant corrasion of the stream 
bed. Corrasion thus cuts the notches or gaps in the several ridges as 
erosion slowly hollows out the valleys between them. 

Deltas. — Upon entering a body of water with little or 
no current, as a lake, a stream deposits its sediment, pro- 
ducing a submerged alluvial cone, which may slowly rise 
to the surface of the water, and be converted into a fan- 
shaped area of low, marshy land. The stream generally 
traverses this new-made land in several radiating channels, 
and by the continued deposit at the several mouths causes 
the land constantly to advance further into the lake. The 
extensive deposit of this kind, accumulating at the mouth 
of the Nile is exceptionally regular in shape, resembling 
the Greek letter delta (a), and from it the name delta has 
come to be applied to all such formations. All streams 
flowing into the ocean would form deltas but for currents 
strong enough to remove the sediment, or subsidence of 
the earth's crust rapid enough to absorb it, as fast as it 
is deposited at the mouth of the stream. Deltas are com- 
mon only in lakes and nearly land-locked seas, for these 
being nearly tideless, are less likely to have strong cur- 
rents ; such are the Mediterranean, the Gulf of Mexico, 
the Arctic Ocean, the North Sea, the east Asia seas, etc. 

The dividing of the main stream into the radiating branches 
which gives the peculiar form to the delta is the result of the varying 
action of the stream at low and high stages of water. Throughout 
its lower course, where the slope is very slight, the stream at low 
water occupies a contracted channel, and the current is just about 
able to move along the load of sediment. At high water, the stream 
spreads out over the whole valley bottom, the low water channel 
marking the deepest water and swiftest current, while on each side 
of the channel the current is much slower, and a great deal of sedi- 



THE DELTA 

and. the 

ALLUVIAL BOTTOM 

of the lower 

IISSISSIPPI RIVER 

and the 

fiELTAor the NILE. 




o F ^ b ° M £ 

LONGITUDE WEST FROM GREENWICH. 



stf- 6 " 
S. £. Pass 
>f ~ * ^ *^S. Pass 

S. W. Pass P0RT EADS 

FOLGER CIN. 



("8) 



WORK OF STREAMS. 



229 



\ Level g of ^ high water % in % Miss, .a River 




{Heights exaggerated 500 times) 
fig- 95- — Profiles across the valley bottom of the Mississippi River. 



ment is deposited and left as a layer of mud when the water sub- 
sides. Now, this deposit is greatest on the banks of the low water 
channel, where the rapid current suddenly changes to a slow one ; 
these banks are thus raised higher than the valley bottom farther 
away from the low water channel. The banks continue to be 
raised in this manner by successive floods until they become so high 
that the weight of the stream when "bank full" bursts the bank at 
some weak point, thus causing a " crevasse," through which part of 
the water in the main stream drains off into the lower land and 
follows down the side of the valley bottom. When this occurs at 
a considerable distance from the mouth, the side stream, after a 
longer or shorter course, as a bayou, generally finds its way back 
into the main low water channel again; but when crevasses occur 
near the mouth of the stream, the bayous form independent mouths, 
and by the corrasion of the soft layers of sediment forming their 
beds, may eventually increase in width and depth until they rival or 
exceed the former main stream in volume. The Atchafalaya Bayou 
of the Mississippi is the highest one having an independent mouth, 
and its divergence, at the mouth of Red River, is therefore called 
the head of the Mississippi delta. 

Estuaries. — When a coast region is sinking with rela- 
tive rapidity the streams are apt to empty into deep and 
narrow bays, fiords or estuaries, formed by the submerg- 
ence of the lower part of the stream valley. Chesapeake 
and Delaware bays and the indentations of the Maine 
coast are such submerged valleys. 



230 



PHYSICAL GEOGRAPHY. 



l/ 



Course.— As a rule, the path of a stream becomes 
more devious as the stream is descended, because the 
declivity and corrasive power usually decrease in that 
direction. A mass of relatively hard material in the bed, 
operates to deflect the stream toward the side composed 
of softer material. A small but steep tributary generally 
tends to deflect the main stream toward the opposite side 
of its valley, for the tributary, being swift, brings down 
particles which the more gentle current of the main stream 
can not carry. A delta-like deposit or bar, therefore, ad- 
vances into the main stream and forces its current against 
the opposite bank, which is rapidly corraded into a loop- 
like bend. A large tributary, during its floods, may in this 
way deposit material entirely across the main stream, 
whose waters are thus dammed back into a long, deep 
pool, while they flow over the deposit as a shallow rapid. 
Whenever a stream is deflected from a straight course, 
the current tends to increase the bend. 

Thus, in any bend of a stream, as BD (Fig. 96), the inertia 
of the current causes it to follow the course of the dotted line; 

hence, the banks at B, Q and D 
are corraded fastest, while sedi- 
mentation frequently takes place 
at F, E, and G, and sand-bars, 
beaches, or mud flats advance 
into the river as the opposite 
bank recedes. Hence, the channel containing the deepest and 
swiftest water is always found close to the concave bank of a stream. 
The effect of this is most marked in the lower course of streams 
where the banks are composed of soft sediment. Figures X, Y, and 
Z indicate progressive states of a bend in such places. In Kthe 
narrow neck of the loop has been cut across. The descent through 
this short "cut-off," being steeper than it is around the loop, the cut- 
off rapidly increases in depth and width by corrasion until it be- 
comes the main channel. The ends of the loop are soon filled with 
sediment, and the crescent shaped lake (Fig. Z) alone remains to 
mark the former site of the river. 




Fig. 96. 



CHAPTER XVII. 

GLACIERS AND LAKES. 

Hast thou entered into the treasures of the snow ? — Job xxxviii : 22. 
Ye shall not see wind, neither shall ye see rain; yet that valley shall be filled 
with water, that ye may drink. — II Kings hi : 17. 

Glaciers. — Wherever more snow falls in winter than is 
melted in summer, the snow tends to accumulate on the 
ground and to move down the slopes. Dry and powdery 
at first, the snow, in passing to lower levels, gradually be- 
comes compacted, by the accumulating weight above and 
the freezing of percolating water from the melting of the 
surface snow, into a white, granular mass called neve. 
At greater depths this mass is compressed into more or 
less transparent ice. Great tongues of this ice creep, far 
below the snow line, down the valleys heading in the 
neve, and constitute glaciers. 

Occurrence. — Glaciers can only form in regions of per- 
petual snow, and in such regions large glaciers can form 
only where the snow-fall is copious. Hence, near the 
equator glaciers are formed only on mountains exceeding 
16,000 feet in height, but they occur at successively lower 
elevations in higher latitudes, and in the frigid zones on 
hills of very moderate elevation. In any latitude glaciers 
are generally largest on those eminences of sufficient 
height which are first encountered by the vapor- bearing 
winds from the sea, and on the sides of these eminences 
which are turned away from the sun ; that is, on the north 

( 23O 




1' t ?' 



„___„___ 



(232) 



GLACIERS AND LAKES. 233 

side in the northern hemisphere, and on the south side in 
the southern hemisphere. 

The Himalaya Mountains, though near the tropic of Cancer, are 
so lofty and so well supplied with vapor by the south-west monsoon 
that they bear immense glaciers ; one has a length of over 35 miles. 
The moderately high mountains of Alaska, and the low mountains 
of Norway, being near the Arctic Circle and well supplied with 
moisture, also bear large glaciers. The Alaskan glaciers are proba- 
bly larger than any others in torrid or temperate zones. On all the 
high peaks of the Sierra Nevada and the Cascade Mountains from 
central California northward, glaciers are found ; on mounts Lyell and 
Dana, Cal., they are less than a 'mile long; Mount Shasta, Cal., has 
one two miles long, while one on Mount Tacoma, Washington, 
is ten miles in length, and surpasses in size and grandeur many 
of the Swiss glaciers. The glaciers of the Alps have been visited 
more than any others. They are found principally about Mount 
Blanc, in France, Monte Rosa, Finsteraarhorn, and the Bernina 
Alps in Switzerland, and the Oetzthaler Alps in the Tyrol. Each 
group has glaciers more than six miles long and a mile or two wide, 
while Aletsch Glacier, on the slope of Finsteraarhorn, has a length 
of 14 miles. The thickness of these glaciers is estimated at be- 
tween 500 and 1,000 feet. All these glaciers, however, sink into in- 
significance when compared with those of the polar regions. These 
form at comparatively low elevations, and, covering the entire coun- 
try with a thick ice sheet, descend into the sea, where great masses 
break off and float away as icebergs. The Humboldt Glacier, of 
Greenland, is thought to be half a mile thick at its sea front. 

Movement. — Glaciers creep downward at a rate vary- 
ing with the slope, the season, and the rain-fall, but sel- 
dom, if ever, at a rate rapid enough to be perceptible 
without measurements. Careful observation has proved 
that the movement of a glacier resembles in many respects 
that of the current of a river. It is faster on steep than 
on flat parts of its bed ; at the surface than toward the 
bottom ; and near the center than at the sides of the sur- 
face. In the curves of its course, the glacier moves fastest 
not at the exact center, but at a point in its surface nearer 
the convex side of the curve. 



234 



PHYSICAL GEOGRAPHY. 




The long undulating arrow follows the line of most rapid motion 
of "Mer de Glace" in the Alps. The amount of movement of the 
surface of the glacier - in inches, per 24 hours in summer- is also 
indicated. 

Fig. 97- 



Along the line on the surface of the Mer de Glace where move- 
ment is fastest, the mean speed is 27 inches a day in summer and 
about one half as much in winter, or about 600 feet a year. Hence, 
this glacier requires more than 25 years to traverse the three miles 
of its length. The thicker Greenland glaciers move more than 30 
feet a day, and almost as fast in winter as in summer. 

If a square block of ice be placed in a mold of any other shape 
and subjected to hydraulic pressure, the ice is crushed to powder, 
which takes the shape of the mold, and immediately re-freezes into 
a solid mass again. The phenomenon is called regelation. The 
amount of pressure required to crush the ice is comparatively slight, 
but increases as the temperature of the ice falls below its melting 
point. This experiment illustrates why the solid ice of a glacier, 
which is brittle rather than plastic, constantly moves downward 
and conforms to the bends and irregularities of its bed as if it 
were a truly plastic substance like wax, thick honey, or thick tar. 
The deepening snow of the neve presses its lower layers into 
solid ice, and at last crushes this ice and squeezes it outward 
down the glacial valleys. But simultaneously with their move- 
ment, regelation unites the particles of crushed ice into a solid mass 
again, which thus transmits pressure to the lower portions of the 
glacier. The faster movement of glaciers in summer is owing to 
the fact that at that season the ice is nearer its melting point, and 
hence yields more easily to pressure than in winter. In addition 
to these movements, the glacier slides bodily forward to a greater 
or less extent, and rapidly corrades its bed. 

Ablation of the Surface. — The surface of the glacier 
is subject to constant lowering by evaporation, and the 



GLACIERS AND LAKES. 235 

entire ice mass, but especially the surface below the snow 
line, loses more by melting in summer than it receives by 
snow-fall in winter. The average lowering, or ablation, of 
the surface of the Mer de Glace is probably six inches 
a day during summer. If, owing to a succession of excep- 
tionally mild winters or hot summers, the amount melted 
exceeds the amount brought down by movement, the 
lower end of the glacier retreats up the valley. If the 
conditions are reversed, the end of the glacier advances 
down the valley. The Swiss glaciers have been advancing 
since 1875. 

Lateral moraines. — The sides of valleys through which 
glaciers descend, being usually steep and in regions of 
great elevation, are exposed to great variations of temper- 
ature and rapid erosion. Large quantities of sand, soil, 
and rock fragments thus find their way to the glacier, 
and are carried by it down the valley. This rubbish is 
specially abundant near the sides of the glacier, where it 
forms long mounds on either edge of the ice. These are 
called lateral moraines. When a* second glacier joins the 
first from a tributary valley, the adjacent lateral moraines 
unite and are carried down the center of the united 
glacier as a medial moraine. 

Each tributary glacier-bearing valley thus produces a medial 
moraine on the main glacier below its junction. The Mer de Glace 
has five medial moraines, one of its tributaries having one and 
another two when they join the main glacier. Medial moraines re- 
main distinct and well marked for some distance, but are gradually 
distributed by the differential motion of the glacier over its entire 
surface. Large quantities of moraine matter protect the ice beneath 
from rapid melting ; thus, medial moraines frequently cover the 
summit of a ridge of ice, while great blocks of stone on the glacier 
are, by the melting of the surrounding surface, left perched as 
"rock tables" on pedestals of ice sometimes 8 or 10 feet high. 

Terminal moraines. — At the end of the glacier, the 
moraine rubbish is dumped upon the ground. If the 



236 PHYSICAL GEOGRAPHY. 

glacier is stationary or advancing, the rubbish accumulates 
to form a curved ridge called a terminal moraine; but if 
the glacier is retreating, the moraine matter is left as a 
coating of approximately uniform depth but very irregular 
surface, covering the ground exposed by the retreating 
glacier. Moraine matter left in this generally distributed 
manner is usually called glacial drift to distinguish it from 
the same material accumulated into terminal moraines. 

Glacial Abrasion of Rocks. — The rocks carried down 
on the surface of glaciers undergo no friction and retain 
their angularity. But vast numbers tumble into the 
crevasses, which at some places open in the glacier to 
great depths, owing to irregularities in the slope of the 
bed or to the differential movement of the glacier. These 
rocks, with others torn from the bed or sides, work their 
way to the bottom, where, pressed down by the over- 
lying ice, they are rasped over the rocky bed by the for- 
ward movement of the glacier ; most powerful abrasion 
results, both of the rocks embedded in the ice and of the 
underlying bed rock. Long, continuous scratches, or 
strice, are indented upon each by the harder particles in 
the other, while the exceedingly fine powder resulting 
from the abrasion acts like emery powder, and gives the 
rock over which the glacier moves a smooth and polished 
surface. As a result of this abrasion, the ordinary V- shape 
of valleys is often changed into a U- shape, the rock of 
their bottom and sides is planed and worn down, and all 
their sharp angles removed ; and where the bed rock is 
soft, it may be hollowed out into deep basins, while, where 
relatively hard, it is worn into smooth, dome-shaped emi- 
nences striated in the direction of ice movement. 

The melting and lowering of the glacier's surface sometimes 
leaves its lateral moraines stranded on the valley sides to mark a 
former height of its surface. The rock tables on the surface, or the 



GLACIERS AND LAKES. 2T,7 

worn and rounded bowlders in the body of a glacier, are also some- 
times left stranded on the steep valley sides among rocks of an en- 
tirely different kind. These are called "perched" or "erratic" 
rocks, and are sometimes left on such precarious foundations that 
the slightest push would apparently be sufficient to set them in mo- 
tion down the slope. When glacial drift is removed from in front 
of a retreating glacier, the bed rock is always found to be smoothed, 
polished, and striated. 

Glacial Streams. — A stream of water always issues 
from the lower end of glaciers. It is derived partly from 
springs, partly from surface waters higher up the glacial 
valley, but chiefly from the melting of the ice. The water 
is charged with an extremely fine light gray silt, formed 
by the constant abrasion of the rocks, which gives it a 
peculiar milky color, and it retains this peculiarity for a 
long time. This sediment forms a deposit of stiff, bluish 
clay, quite impermeable by water, and in marked contrast 
to the yellow mud deposited by rivers generally. 

Former Extent of Glaciers. — Indications of glacial 
action on the valley sides high above the present surface 
of glaciers, and the occurrence of old terminal moraines, 
drift, polished rock surfaces, erratics, etc., not only far 
beyond the end of glaciers, but over vast regions hundreds 
and even thousands of miles from any existing glacier, 
prove that at a comparatively recent period in the past, 
glaciers had a much greater extent than at present. The 
whole northern half of North America and Europe are 
thus glaciated. The mountain summits are striated and 
polished, and the lowlands are deeply buried under accu- 
mulations of drift. This region in each continent must 
have been covered, as Greenland is to-day, by an immense 
sheet of ice, so thick that only the highest mountain 
peaks protruded above its surface. Many circumstances 
indicate that the movement of these continental glaciers, 
in Europe, was outward in all directions from the high- 




(238) 



GLACIERS AND LAKES. 



239 



lands of Norway, while in the United States the move- 
ment was from the Canadian Height of Land. 



The southern limit of this vast glacier can not be determined 
exactly. A terminal moraine (Fig. 98), from one to several miles 
broad, has been traced west from Cape Cod through the inter- 
vening states into the Dakotas and the Dominion of Canada, and 
forms a limit which the ice certainly reached. But glacial drift ex- 
tends many miles south of this moraine in some localities. 



LAKE REGION 

bounded by a 

RMINAL MORAINE 

xtending fj.om Cape Cod 
west through Dakota. 




Fig. 98. 



Effects on the Relief of the Land. — The thickness 
of this glacial drift has been ascertained in many localities 
in the United States, and has been found to vary from a 
few feet to four or five hundred feet. It is generally 

P. G.— 14. 



24O PHYSICAL GEOGRAPHY. 

thickest in the vicinity of the moraine, and is generally 
thicker in the valleys than on the higher land. Almost 
all the valleys in the drift regions are more or less filled 
with drift gravel, sand, and the peculiar blue clay of 
glacial origin. The drift very generally fills not only the 
bottom of valleys, and forms the bed of the stream, but 
frequently forms a series of terraces along either side of 
the valley to a height of several hundred feet above the 
stream, which indicate the amount of drift removed by 
the stream since the glacial period (Fig. 99). 



Fig. 99. 

Cincinnati is built upon two such drift terraces of the Ohio valley, 
at elevations of 65 and 130 feet above low water in the river. The 
bed of the Mississippi River at La Crosse and Prairie du Chien is 
more than 100 feet above bed rock, and of the Rock River at Janes- 
ville, Wis., more than 250 feet. 

The drift deposit, especially where greatest, in the region from the 
moraine northward, entirely blocked up and buried many old val- 
leys, destroying the ancient drainage lines, and, by its own irregu- 
larities, presented a new and peculiar surface, formed in disregard 
of drainage demands. In these irregularities water collected, giving 
rise to innumerable lakes (Fig. 98), which are the special feature of 
the region north of the moraine in both America and Europe. 

Formation of the Great Lakes. — While thousands 
of small lakes in and north of the moraine region occupy 
simple depressions in the surface of the drift, the larger 
lakes, including the American Great Lakes, probably oc- 
cupy old preglacial valleys of atmospheric erosion, which, 
however, were modified by movements of the earth's 
crust and greatly broadened and deepened by the abra- 



GLACIERS AND LAKES. 24I 

sion of the glaciers themselves, the bulk of the drift south 
of these lakes being the material so removed. 

There are indications which render it not unlikely that the pre- 
glacial valleys of lakes Michigan and Superior were tributary to the 
Mississippi ; the Michigan valley possibly south-westward through 
Illinois, where an ancient valley, completely obliterated by a depth 
of 200 feet of drift, has been traced ; and the Superior valley, either 
westward in the vicinity of St. Croix River, where the drift is very 
thick, or southward by some deep, narrow, and as yet undiscovered 
valley, across the upper peninsula of Michigan into the Michigan 
valley. The Huron-Erie-Ontario valley probably found an outlet 
through the St. Lawrence. There are strong indications of the ex- 
istence of a deeply buried and concealed valley connecting these 
lakes. The filling of these natural channels at places divided the 
valley into separate basins, and forced the waters in each basin to 
seek a new channel at the lowest point of its water-shed. 

Lakes. — Whenever the water of a stream system, in 
its downward course over the land, meets an obstruction 
to its further advance, its current is checked, and it tends 
to accumulate on the upper side of the obstruction to form 
a pond or lake. The constant addition of water from the 
stream tends to raise the level surface of the lake to the 
lowest point at which the water can escape through or 
over the obstruction to form an outlet. As the surface of 
the land is generally sloping and seldom precipitous, a very 
slight rise of the lake usually occasions a great increase 
both of its width and length, and hence of the water sur- 
face exposed to evaporation. It thus sometimes happens 
that before the lake rises to a point at which it can find 
an outlet, the increased evaporation from its surface equals 
the amount of water constantly added by tributaries. 
In this case the water surface can rise no higher, and a 
lake lying in an inland basin— that is, a lake having no 
outlet — is formed. Thus, lakes may be divided into two 
classes: (1) those having outlets, and (2) those having no 
outlets. It is an almost invariable rule that lakes with 



242 PHYSICAL GEOGRAPHY. 

outlets contain fresh water, while lakes without outlets 
contain salty or bitter and undrinkable water. 

Fresh Water Lakes. — Since the water of all streams 
contains more or less mineral matter in solution, and since, 
upon evaporating, water leaves all impurities behind, the 
greater relative evaporation from the wider lake surface 
tends to increase the proportion of dissolved impurities in 
the lake water ; hence, lake water usually contains more 
matter in solution than the average water of its tributary 
streams. When the lake has an outlet, however, this 
difference is so slight that the taste of the lake water is 
not usually affected, and the difference does not increase 
beyond a certain point, for the impure lake water is con- 
stantly escaping by the outlet, while purer water is con- 
stantly entering the lake through its tributaries. 

Salt Water Lakes. — In lakes having no outlets, the 
constant loss of pure water by evaporation, and the con- 
stant addition of the small proportion of mineral matter 
dissolved in the tributaries, causes a constant increase of 
the mineral matter in the lake water, until, eventually, it 
becomes saturated with some mineral, that is, can hold no 
more of this mineral, though other minerals present may 
continue accumulating. Further accessions of the satu- 
rating mineral are deposited in solid crystals on the lake 
bottom. Long before the water becomes saturated, the 
mineral is in sufficient quantity to have imparted to the 
water its peculiar taste, if it has any. The proportion of 
dissolved minerals in various waters is here given: 

1 barrel of average fresh water contains about xwo °f a <!*• °f minerals. 
1 " " " ocean water " 3 quarts " " 

1 " " " Dead Sea water " " 22 " " " 

The kinds of mineral in any salt lake, and the rela- 
tive amount of each, depend partly, of course, upon the 
character of the rocks composing its tributary basin, and 



GLACIERS AND LAKES. 



243 



partly upon the effect which different minerals in solution 
have upon each other. Some minerals limit the ability of 
water to hold in solution certain other minerals, while 
they entirely prevent the water from holding in solution 
still other minerals. The principal minerals in solution, 
and their proportion (by weight) are given below. 



Bodies of Salt Water. 



Kara Booghaz (gulf of Caspian) 
Dead Sea (average) .... 

Great Salt Lake 

Mediterranean Sea (average) . 

Open Ocean 

Black Sea 

Open Caspian Sea 

Average proportions, 



Total in 

Solution. 

Lbs. per 1,000 



285 

243 
ISO 

38 

35 
18 

13 



Common 
Salt. 



29% 
26 

79 
78 
78 
79 
63 



Chlorides 

and 
Sulphates 
Magnesia 
and Lime. 



70 
IO 
15 

19 
16 

29 



All 
Other. 



4% 
4 
11 

7 
3 
5 



62% 



32% 



6% 



Bodies of Fresh Water. 


Total in 

Solution. 

Lbs. per 1,000 


Common 
Salt. 


Carbonates 
Lime and 
Magnesia. 


All 
Other. 


Average River Water .... 


1B2 

T00TJ 
145 
Tooo 


3% 
4 


64% 
78 


33fo 
18 



Average proportions, 



lM 71% 25^$, 



It might be expected that the proportion of the different minerals 
in salt lake water would be the same as in its fresh water tributaries, 
but simply increased in quantity. The above table indicates, how- 
ever, that 94% of the mineral in the salt waters consists of com- 
mon salt, and the chlorides and sulphates of magnesia and lime, 
while in the fresh waters these minerals form less than 30%, since 
71% is composed of the carbonates oi lime and magnesia. Not only 
are aquatic plants and animals constantly robbing water of its dis- 
solved carbonate of lime, but when the very slight quantities of 
chlorides and sulphates usually found in fresh waters have accumu- 
lated to a certain extent in the lake or sea, they cause the water to 



244 



PHYSICAL GEOGRAPHY. 



deposit almost all of its carbonates, thus leaving the salty and 
bitter chlorides and sulphates in excess in the solution. When 
certain chlorides and sulphates accumulate still more, they act in a 
similar manner upon each other, and thus cause a deposit of much 
of the common salt (chloride of sodium). Thus, owing to the large 
proportion of chloride of magnesia in the water of Kara Booghaz 
and the Dead Sea, it can not hold nearly as much salt as that of 
Great Salt Lake, in which less chloride of magnesia has as yet ac- 
cumulated. This is indicated graphically in Fig. loo. This diagram 
also indicates that the Dead Sea, Great Salt Lake, and parts of the 
Caspian — all of them simply lakes without outlets — contain a vastly 



KARA BOOGHAZ 
DEAO SEA 
GT.SALT LAKE 
MEDITERRANEAN 
OPEN OCEAN 
BLACK SEA 
OPEN CASPIAN 
RIVER WATER 
LAKE MICHIGAN J 




Fig. ioo. — Amount and Proportion of Dissolved Minerals in Various Waters 



greater proportion of mineral matter in solution than the water of 
the ocean or of its arms — the Mediterranean and Black seas. As 
the surface of both the Caspian and Dead seas is considerably 
below sea-level, it is thought they may at one time have been arms 
of the ocean, which have been separated from the ocean by the up- 
heaval of the intervening region. But even if this is the case, by 
far the greater part of their intense "saltness" is due to the same 
causes that would gradually convert any fresh water lake into a salt 
lake if its outlet were permanently closed. 

The checking of the current of tributaries upon en- 
tering a lake causes a deposit of more or less of the solid 
particles held in suspension. Hence, all lakes are being 
gradually obliterated, not only by the corrasion of the out- 
let channel, which tends to lower the lake surface, but by 
the deposit of sediment, which tends to fill up the lake 
basin. On account of this deposit, the water of every 
lake and of its outlet is clearer than that of its inlet 
streams. Relatively large lakes usually contain very clear 



GLACIERS AND LAKES. 245 

water, since they have the slowest current and the least 
amount of matter in suspension. When, in addition to 
the large relative volume of the lake, the water of the 
tributaries is clear, as is usually the case in regions of hard 
rock, the water of the lake is exceptionally limpid and 
transparent. 

Lakes fed by streams flowing from glaciers — as Geneva, Mag- 
giore, Como, and many others — have usually a beautiful blue color, 
but really their water is less clear than that of lakes of large relative 
volume fed by ordinary streams. Much of the silt of glacial 
streams is so fine that the slight current of the lake can hold it in 
suspension. These fine particles in suspension, by reflecting only 
the blue rays of light, give the water its peculiar color, just as the 
fine particles of the air give the sky its azure tint (see page 104). 

Effect on Floods. — Floods in tributary streams, upon 
entering a lake, are spread out over its relatively great area, 
and thus do not materially raise its level surface. Hence, 
neither lakes, nor streams issuing from them, are subject 
to such great variations of level between low and high 
water as are streams tributary to lakes, or those in whose 
course no lakes occur. 

Thus, there are no such great floods in the Great Lakes or their 
outlet, the St. Lawrence, as occur annually in the Ohio, Missouri, 
and other rivers whose courses contain few or no large lakes. The 
mean annual fluctuation between high and low water in the Great 
Lakes is less than \% feet. In the Ohio, at Cincinnati, it is more 
than 50 feet. 

Temperature of Lakes. — The temperature of the sur- 
face water in lakes varies with the seasons, but on account 
of the great specific heat of water it does not vary so rap- 
idly, and hence not to so great an extent, as that of the 
overlying air or the neighboring land surface. During the 
summer the water is generally cooler than the air, which 
it therefore tends to cool, while during the winter the 
water tends to keep the adjacent air warm. If the winter 



246 PHYSICAL GEOGRAPHY. 

temperature at the surface of fresh water lakes falls to or 
below that of the maximum density of water (p. 25), the 
temperature at the bottom is 39 , and, if the lakes are deep, 
it remains constant throughout the year. From this bot- 
tom water in such lakes, the temperature increases to that 
of the surface in summer, but may decrease to a surface 
temperature of 3 2° in winter. 

Distribution, — Lakes are much more numerous in 
some regions than in others. As a general rule, lakes are 
more numerous near water-sheds than elsewhere. Near 
water-sheds, streams are short and small; hence they 
carry but little sediment, and possess little power either to 
corrade lake-forming obstructions or to fill up and obliter- 
ate lake basins. There are five kinds of regions where 
lakes are particularly abundant, the lakes being generally 
fresh if the rain-fall is abundant, but salt if the rain-fall 
of the region is scanty. 

( 1 ) Glaciated regions, or those whose surface has been cov- 
ered with irregularities by the abrasion or drift -deposit of former 
glaciers. North-eastern America and north-western Europe are 
such regions, the old terminal moraine forming in the United States 
a sharp boundary between a vast lake region on the north and a 
comparatively lakeless region on the south (Fig 98). 

(2) Mountainous or hilly regions generally. — In these regions 
the valleys are steep and narrow. The one quality favors the ero- 
sion of large masses from the sides of the valley ; the other permits 
a comparatively small quantity of material to make a high obstruc- 
tion across the valley ; consequently, mountain lakes are generally 
very narrow and very deep. If the five Great Lakes in the com- 
paratively flat portion of the United States be compared with the 
five Alpine lakes, Geneva, Constance, Como, Maggiore, and Garda, 
it will be found that the Great Lakes have an average width of one 
fourth, but the Alpine lakes of only one ninth of their lengths. The 
average of the greatest depths in the Great Lakes is 705 feet, and 
of the Alpine lakes 1,491 feet. 

(3) Non-glaciated, basin-shaped plateau regions having a 
copious rain-fall. The most remarkable of these extends south- 



GLACIERS AND LAKES. 



247 



ward from Abyssinia in eastern Africa. It contains a number of 
lakes which rival our Great Lakes in size. 

(4) Regions of scanty rain-fall in general. — These lakes are 
seldom large; they generally have no outlet, and hence contain salt 
water, and many of them are entirely evaporated during the drier 
seasons of the year. Almost all the salt lakes of the world occur 
in regions having a mean annual rain-fall of less than 10 inches 
(see chart, page 76). Lakes are rather numerous in these regions 
for the same reason that they are numerous near water-sheds — the 
supply of drainage-water being small and often intermittent, the 
streams make little progress in cutting away lake-forming obstruc- 
tions or filling up lake basins. Among the largest salt lakes are 
Caspian, Aral, and Dead seas and Balkash Lake of Asia, and Great 
Salt Lake of Utah. Though they lie in nearly rainless regions, 
they all receive tributaries from regions of more copious rain-fall. 
The Caspian Sea is five times as large as Lake Superior, and 
though it receives the Volga and five smaller rivers, the evaporation 
from its vast area is so great that its surface lies 85 feet below the 
level of the ocean, and 171 feet below the lowest point in its water- 
shed. The main body of the sea is only brackish, for the great 
shallow and nearly land-locked gulfs on its eastern coast — as the 
Kara Booghaz, which is nearly half as large as Lake Erie — lose so 
much water by evaporation that a constant current flows into them 
and acts as an outlet to the main sea. The water of these gulfs is 
much Salter than ocean water on the same principle that the water of 
the Mediterranean is slightly so. (See page 144.) 

(5) Low and sandy sea-coasts are frequently fringed with shal- 
low, brackish lakes or lagoons. They occur along the whole east 
coast of the United States south of Cape Cod. They are separated 
from the ocean by a narrow beach of sand, and receive the drainage 
of the coast region through small streams. The narrow beach is 
the joint result of the sediment of the small streams and the sand 
piled up by the sea waves and the wind. * 



CHAPTER XVIII. 

MOUNTAIN STRUCTURE AND LAND SCULPTURE. 

Every valley shall be exalted, and every mountain and hill shall be made low : 
and the crooked shall be vzade straight, and the rough places plain : and the glory 
of the Lord shall be revealed. — Isaiah xl : 4. 

Mountain Formation and Sculpture. — The repeated 
uplifts and subsidences of the earth's crust which have 
resulted in the gradual formation of the continental 
plateau, have in general thrown the rock strata which 
compose the land, into a series of wave-like undulations. 
In some extensive regions the undulations are so broad 
and low that the curvature is quite imperceptible, the 
strata lying apparently horizontal or having a very gentle 
and uniform slope over great areas. This, in general, 
is the position of the strata composing plains and plateaus. 
In the long and comparatively narrow mountain regions, 
however, which traverse each of the grand divisions, the 
undulations are " much narrower and higher. In some 
regions the strata have been thrown into a succession of 
huge open waves, while in others the waves have been 
crowded together into a series of closely compressed folds, 
so that the strata stand directly on end or are even over- 
turned, older rocks lying on top of newer ones. Long 
faults or fractures where the strata have slid up or down 
or sideways hundreds and even thousands of feet, are very 
numerous in mountain regions. The rocks are generally 
more or less completely metamorphosed, hard gneiss and 
massive granite often occupying large areas, while dikes 
(248) 



MOUNTAIN STRUCTURE. 249 

and hardened outflows of lava are almost invariably found 
in some parts of the region. 

The atmospheric agents, and streams of running 
water, which are constantly disintegrating, removing, and 
hence lowering all parts of the land surface, are especially 
energetic and rapid in their action in these regions of high 
elevation, and steep and broken strata. A covering of 
rock, probably many thousand feet thick, has been thus 
removed from all parts of every mountain region. It is 
believed that a thickness of five miles has been so removed 
from much of the Appalachian chain, and that at least 
one mile has been eroded from the entire region between 
the Rocky and Wasatch mountains. 

This enormous erosion has seldom been uniform, 
however, over a mountain region. Other things being 
equal, it has been greatest where the elevations were 
highest, the slopes steepest, and the rocks softest. The 
elevated tops and steep sides of the folds of the strata 
have thus generally been most deeply eroded, and hence 
older rocks are generally exposed along the crests than 
along the troughs of the folds. On account of this great 
but unequal erosion, the crests of the mountains do not 
always conform to the crests of the folds ; but, in general, 
mountain ranges are simply the projecting remnants of 
those portions of folds, which, on account of the greater 
hardness or the more stable position of the strata, have 
been best able to resist erosion. The great folds and 
faults in the earth's crust have therefore determined the 
direction and general position of mountain ranges, but 
the shape of a range — every peak, ridge, spur, valley, and 
canon — is directly and entirely due to erosion. 

Mountains of simply folded strata. — The simplest 
mountain chain is that which is carved from a single 
broad fold of very thick strata; such is the Uinta range 




(250) 



MOUNTAIN STRUCTURE. 



25 1 



In the background the Uinta 
fold is supposed to have i 
mained zineroded, while 
the foreground sho 
the Uinta Mount- 
ains as they exist. 




The dotted line 
shows the part 
f fold remooed. 



'OLDEST (mEtXmORPHIC) 



U I N 

Ti-ra-kau Plateau 



T A MOUNT 
Crest 



by erosion, 

NORTH 




Fig. ioi. 



of Utah (Fig. 101). Although a thickness of y/ 2 miles 
of rock has been eroded from this range, the deeply buried 
rocks now exposed along its crest have been but little 
changed by metamorphism. The three main ranges of 
the Rocky Mountains in Colorado have been carved from 
a series of three broad, flat, Uinta-like folds; but in this 
instance the underlying metamorphic granite has been 
exposed by the erosion, and forms the mountain crests, 
while the upturned remnants of the unchanged stratified 
rocks which once covered the higher parts of the range 
form a series of "hog backs" or foot-hill ridges along its 
base (see Rocky Mountain section opposite). 

Mountains of closely folded strata. — More fre- 
quently mountain chains have been carved from an up- 
heaval consisting of a greater number of more pronounced 
folds. When the structureless granite has not been ex- 
posed by erosion, the projecting edges of the harder 



252 PHYSICAL GEOGRAPHY. 

strata composing the folds form a well defined and regular 
series of long, parallel ridges of nearly uniform height, 
such as constitute the Appalachian chain of the United 
States (see page 250), and the Jura Mountains of France 
and Switzerland. In many of the larger chains of the 
world, however, the great thickness of stratified rock re- 
moved from the top of the folds has exposed either the 
granite or equally hard gneiss, i. e., granite in which some 
of the lines of stratification are still obscurely visible. The 
section across the Alps (page 250) may therefore be re- 
garded as typical of all great mountain chains. 



/ SNOW MASS PEAK 

: . INVERTED 

=OLDER ROCKS^- "„N 




Granite' ^ ssese^^^rc-^asr^^- 

IN ELK MOUNTAINS, COLORADO. IN THE ALPS, SWITZERLAND. 

Fig. 102. 

The greater folds of most mountain regions are corrugated by 
many minor plications. The portions of these minor foldings left 
by erosion often render the structure of certain regions exceedingly 
complicated, as indicated in Fig. 102. In other localities the struct- 
ure is entirely concealed by great outflows of lava thousands of 
feet thick and hundreds of square miles in extent; such is notably 
the case in the Cascade Range in the states of Oregon and Wash- 
ington. 

Mountains Produced by Faultings of the Strata. — 
The numerous and nearly parallel ranges of the Great 
Basin, western Arizona, and northern Mexico are some- 
what different in structure. The region between the Sierra 
Nevada and the Wasatch Mountains, and extending from 
Idaho to Mexico, is composed of very gently folded rocks 
deeply buried in places by extensive outflows of lava. A 
series of nearly parallel fractures, hundreds of miles long 
and fifteen to thirty miles apart, traverses this entire 



AGE OF MOUNTAINS. 253 

region and divides it into long, narrow blocks. Many 
facts prove that the whole region was once more elevated 
than at present, but has subsided thousands of feet, and 
during the subsidence the blocks have been tilted side- 
ways. The uptilted side of these blocks, carved by sub- 
sequent erosion, forms the. isolated mountain ranges of the 
region (see section of Basin Ranges, page 250). 

The rate of mountain upheaval, either by folding or 
faulting, is always an exceedingly slow process, the rocks 
giving way a few inches, or at most a few feet at a time, 
and at very long intervals, as the stresses accumulate. It 
is certain that such movements are taking place at present 
in many mountain regions, notably throughout the great 
and nearly continuous highland on the convex side of the 
continental plateau. Hence, notwithstanding the enor- 
mous thickness of strata eroded from their tops, many 
mountains may never have been higher than they are at 
present, erosion having planed down the surface about as 
fast as it was upheaved. Many of the older mountains, 
however, as the Appalachians and the mountains of 
northern Europe, in which the upheaval has probably long 
since ceased, have probably been greatly lowered by the 
subsequent erosion. 

The extreme slowness of upheaval is conclusively proved in 
many cases by river gorges, canons, or water gaps, cut directly 
across the mountain range (page 226). This is plainly seen in the 
Uinta Mountains. The great Uinta fold rose directly across the 
upper course of the Colorado River, but it rose no faster than the 
river deepened its channel ; hence, the river has not been deflected 
from its course, but flows through the mountain m a deep canon, 
which the river cut as the fold rose. 

Age of Mountains. — Most mountain chains have been 
upraised by a succession of these gradual uplifts, separated 
by long ages of rest or subsidence. The time of the up- 
heaval which left the region permanently above the sea 



254 PHYSICAL GEOGRAPHY. 

determines the "age" of the mountain. The Appa- 
lachian chain was permanently raised above the sea before 
the close of the paleozoic era; the Sierra Nevada at the 
close of the Jurassic period; the Rocky Mountains at the 
close of the cretaceous ; and the Coast Ranges as re- 
cently as the close of the miocene. Hence, the Appa- 
lachians have been subjected to much longer erosion than 
the mountains of the west. The difference in the length 
of time that various ranges have been exposed to erosion 
may partly account for the fact that the oldest mountain 
chains are never very high, while the highest ranges are 
invariably among the youngest. The high and nearly 



"^ N £""' 1 Archaean and Granite ] ~2 3~ 

Age of rocks. 7 — Archaean, 2 -Silurian, S — Carboniferous, 4 — Triassic, 5 — Jurassic, 

6 — Cretaceous, 7 — Miocene- (most recent)- 

Dotted lines indicate shape of eroded portion of fold. Vert, scale 6 times horizontal. 

Fig. 103. 

continuous ranges on the convex margin of the conti- 
nental plateau, and the Alps and the mountains of Vene- 
zuela are all of recent upheaval, while most of the 
detached and lower ranges near the concave or Atlantic 
margin are much older. 

The age of mountains is determined by the ages of the uncon- 
formable strata involved in the folds. A section across the Black 
Hills of S. Dakota illustrates the process (Fig. 103). There are two 
lines of unconformity in this range : between the ancient archaean (1) 
and the overlying Silurian (2) rocks, and between the comparatively 
recent cretaceous (6) and the overlying miocene (7). This indicates 
the history of the range, which is typical of mountain ranges in 
general. During archaean time the rocks of the nucleus were deposited 
beneath the sea as horizontal strata, were then folded and metamor« 



AGE OF MOUNTAINS. 255 

phosed, and the strata separated by great protrusions of granite. 
They were elevated above the sea, and the tops of the folds removed 
by erosion. At the close of archaean time began a long continued 
subsidence below the sea again, during which the successively more 
recent rocks of the Silurian, carboniferous, triassic, Jurassic, and cre- 
taceous periods were deposited in horizontal strata above each other 
on the upturned edges of the archaean strata. Then another period 
of upheaval ensued, which gradually bent all the horizontal strata 
up into the great flat arch shown by the dotted lines, and raised them 
permanently above the sea. This upheaval took place soon after 
cretaceous time, for the unconformable strata (7) contain fresh water 
fossils of middle tertiary age, and are composed of material eroded 
from the higher parts of the arch, and must have been deposited in 
an old fresh water lake about its base. Hence, these mountains 
are of eocene age, since the uplift from which erosion is still carv- 
ing them took place during the first period of the tertiary era. 

Thickness of Sediments. — The ragged and upturned 
edges of strata in all mountain regions prove that before 
upheaval and erosion the thickness of stratified rock in 
these regions was exceptionally great. In the plicated 
Appalachian region the stratified rocks were eight miles 
thick, while in Indiana, where the same strata are nearly 
horizontal, they are known to be less than one mile thick. 
Now, the region where sediment is to-day accumulating 
fastest is a comparatively narrow belt of sea bottom along 
the margin of the continental plateau. The incessant 
erosion of material from the land, and its constant deposit 
in this "littoral" belt, disturbs the subterranean equality 
of pressures (page 151), and causes some regions of the 
land to rise, while the narrow marginal region of sea bot- 
tom subsides as weight gradually accumulates upon it. 
Thus, during the lapse of ages, sediment many miles thick 
may be deposited in these regions, the water remaining 
comparatively shallow all the while. 

No theory of mountain upheaval yet advanced is 
complete and satisfactory. The long and narrow shape of 
p. G.-15 



256 PHYSICAL GEOGRAPHY. 

mountain regions, their rough parallelism with coast lines, 
and the comparative proximity of all the younger and 
higher mountains to the sea, together with the fact that 
sedimentary rocks are exceptionally thick in these regions, 
are all to be easily explained by supposing that mountain 
chains mark the general position of former marginal belts 
of sea bottom, which, after a longer or shorter period of 
subsidence, underwent great but gradual upheaval to 
form an elevated border to the previously existing land. 
It is generally believed that the great mountain systems 
have been formed by the successive uplifts of such mar- 
ginal regions, and there are indications that the upheaval 
and accompanying plication, folding and faulting of the 
strata are primarily due to the gradual increase of subter- 
ranean temperature, and the consequent resistless expan- 
sion of the deeply buried rocks in such localities. But 
satisfactory reasons have not yet been found to account for 
the vast and relatively local variations of temperature in 
these thick accumulations of sediment, required to convert 
them from regions of subsidence into regions of excep- 
tionally great elevation. 

It is not impossible that the accessions to the width of the conti- 
nental plateau, caused by the upheaval of the marginal belt of sea 
bottom, have taken place alternately on its two sides. The most 
recent accessions have been on the convex or Pacific side, which in 
general seems to be still rising, and in consequence of its elevation 
the streams and sediment of most of the land have been directed 
into the marginal region of the concave or Atlantic side, where, 
therefore, the foundations of the great mountain chains of the 
future are possibly now being laid. Indeed, the site of such a 
chain is possibly already marked out by the Lesser Antilles 01 
Windward Islands. 

Land Sculpture. — While erosion has been greatest in 
mountain regions, the whole surface of the land has prob- 
ably been lowered many hundreds, or even thousands, of 



LAND SCULPTURE. 



257 



feet by the rains, frosts, and winds of countless centuries; 
and the position and relative hardness of the different 
rock strata, by influencing their resistance to erosion, have 
determined the alternation of hill and valley in the low- 
land regions of comparatively horizontal rock strata. 

In nearly horizontal strata, the corrasion of the main 
valleys leaves the intervening region as broad, flat-topped 
plateaus. The multiplication and deepening of tributary 
valleys eventually cuts up the plateau into irregular series 
of hills, with rounded outlines and nearly uniform height. 
The western part of the Appalachian table-land from New 
York to Alabama is thus cut up, while the eastern part 



reOOOrfTVABOVE-SEA- 



MESA OF SAN MATEO MTS. 




W//A - Hard Laua H~| = Much softer strata 

Fig. 104. — Lava-capped Mesas in North-western New Mexico. 

still retains more of its true plateau character. Where 
the different strata vary greatly in hardness, the outline 
of the hills is more -regular, the harder strata forming 
lines of cliff along their tops and sides. In the regions 
of lava outflows in the West isolated, flat-topped " mesas" 
or "table mountains" are numerous, the hard lava resist- 
ing the erosion which lowers the surrounding regions (Fig. 
104). Where the horizontal strata are very soft, as in the 
numerous extensive regions of "bad lands" of the West, 
erosion has carved the surface, into, great numbers of 
steep, isolated cones and pinnacles, whose soft sides are 
scored by the rain streamlets into countless straight 
grooves. These upright flutings, together with the lines 
of horizontal bedding, suggest regularly laid masonry, and 



258 



PHYSICAL GEOGRAPHY. 




Fig. 105. — Forms of Erosion in Washakie Bad Lands, Wyoming. 

give the isolated masses, when seen at a little distance, 
the aspect of a gigantic city in ruins (Fig. 105). 

Gently Inclined Strata. — A common effect of erosion 
on gently inclined strata is shown in the diagram (Fig. 
106). A succession of long, parallel lines of cliff are 




Hard Strata 



Soft Strata 
Fig. 106. 



= Original Surface 



formed, separated by plains often many miles in width, 
which drain toward the base of the cliffs above. The 
surfaces of the plains are the harder strata which form the 
crest of the terminating cliff. The gradual breaking away 
of fragments slowly carries the lines of cliff backward down 
the incline of the strata. The wash of rain torrents 
carves the face of the cliffs into a succession of deep bays 



LAND SCULPTURE. 



259 



with bold promontories between. The widening of adja- 
cent bays frequently detaches the ends of the promontories, 
which, by the recession of the main cliff, are thus left as 
isolated "buttes" far out upon the plain below. Gradual 
weathering eventually disintegrates the hard capping 
strata of the buttes, and the butte rapidly disappears. 
Such lines of cliff, sometimes 2,000 feet high and hun- 




Fig. 107. — Vermillion Cliffs, Utah, showing Outlying Buttes. 

dreds of miles long, are very common in the Colorado 
Plateau district (Fig. 107). When some of the cliff- 
forming strata are conglomerate (rock composed of great 
bowlders cemented together), its disintegration and reces- 
sion frequently leave detached high, slender "needles" 
of soft strata, capped by a hard bowlder. These needles 
are frequently hundreds of feet high, and stand until the 
gradual weathering of the soft strata diminishes the sup- 
port of the capping bowlders, which at last topple over 
and the rains rapidly reduce the needles. Other pecu- 
liarities in the relative hardness of the various rocks cause 
the outlyers of the receding cliffs to weather into great 
natural arches and other fantastic forms (Fig. 108). 

In sharply folded strata, the tendency of erosion is 
always to wear away the top of the fold most rapidly, for 



260 



PHYSICAL GEOGRAPHY. 







Fig. 108. — Various Fantastic Forms of Erosion. 



not only- is the rock in that locality most apt to be greatly 
fissured, cracked, and weakened by the strains in folding, 
but the position of the strata is an unstable one, for when 
erosion has excavated a valley in the trough of a fold (Fig. 

109) the inclined strata on the 

sf^cfsB^- -i£^§|t^v^_ sides thus deprived of their 

support tend to move down- 
ward as a land-slide, which 
partially or wholly fills the valley and delays the erosion 
of the trough, while erosion proceeds with undiminished 



LAND SCULPTURE. 



26l 



activity on the crests and sides of the folds. This more 
rapid lowering of the surface under the crests than under 
the troughs of the folds is very conspicuous in all mountain 
regions of sharply folded strata, and the process has gone 
so far in the older mountains, such as the Appalachian, 
that the present ranges usually occur either along the 
troughs, or are composed of specially hard strata on the 
inclined sides of the folds, while in younger mountains, 
such as the Juras, the tops of the folds have not yet been 
so greatly lowered ; but before they have suffered erosion 
as long as the Appalachians, the present mountain sum- 



THE OLD APPALACHIAN RIDGES OF PENNSYLVANIA 




Fig. no. 

mits will probably be valleys, while the site of some of 
the present valleys may be occupied by ridges. In old 
mountains, therefore, such as the Appalachians and the 
mountains of northern Europe, most of the strata which 
occupied an unstable position have been removed, and 
consequently land-slides are of rare occurrence. In 
younger mountains, however, such as the Alps and the 
Sierra Nevada, many of the strata still remain in the un- 
stable position, and extensive land-slides are numerous 
and often very disastrous. 

Canoe-shaped Valleys. — Though many miles long, 
the individual folds of mountain regions are seldom nearly 
as long as the whole disturbed region in which they occur. 



262 



PHYSICAL GEOGRAPHY. 




Fig. in.— Formation of Canoe-shaped Valleys. 



In reality, each fold is a greatly elongated dome. A 
sketch of such a dome-fold is shown in Fig. 1 1 1 (A), 
its internal structure being shown at B. The hard strata 
left projecting by the erosion of such a fold form mountain 
ridges which gradually approach and unite at either end of 
the dome-fold (C), thus inclosing one of the lozenge or 
canoe-shaped valleys so common in central Pennsylvania. 
Frequently several minor folds are pressed closely together, 
and this corrugated surface carried up into a great dome- 
fold (D). In this case, the erosion of the fold would leave 
the projecting hard strata in the form of a mountain ridge 
having curious zigzags in its trend (if). 

Water Gaps. — The drainage of such confined valleys 
usually escapes through the inclosing mountain by a 
narrow notch or zvater gap. These water gaps have been 
cut entirely by erosion, but frequently at a point where 
the hard strata forming the mountain rim have been broken 



LAND SCULPTURE. 263 

and slightly displaced by a fault. Such a slight displace- 
ment has determined the position of the Delaware Water 
Gap. 

The granitic crests of high mountains generally 
weather into a very jagged and irregular outline, — sharp, 
high peaks, alternating with relatively low passes. This 
peculiarity led to the name sierra — the Spanish word for 
saw. It is largely due to the absence in highly metamor- 
phic rocks of the lines of stratification, which, by direct- 
ing percolating water into definite channels, cause the 
stratified rocks to disintegrate with an approach to reg- 
ularity. 



CHAPTER XIX. 

EARTHQUAKES. 

Then the earth shook and trembled ; the foundations also of the hills moved and 
were shaken. — Psalm xviii : 7. 

Earthquakes. — The constant wearing away of parts of 
the earth's surface by erosion, and the building up of the 
other parts by deposit, causes a very slow but incessant 
change in subterranean pressures and temperatures (page 
186). These changes in pressure, and the tendency toward 
expansion or contraction accompanying these changes of 
temperature, place the deeply buried rocks every-where in 
various states of stress or strain. No region is exempt from 
such stresses. For years or centuries the stresses accumu- 
late until they become greater than the rocks can bear. 
A sudden but slight movement of the strata then occurs, 
which relieves the stress, and for another long period the 
rocks remain practically stationary, while stresses again 
accumulate. Thousands of such slight movements, dis- 
tributed over tens or hundreds of thousands of years, 
result in great upheavals or subsidences of the earth's 
surface, with faultings, flexures, or plications of the 
strata. The shocks or jars of each of these slight but 
sudden movements in deeply buried strata are rapidly 
transmitted through the rocks in all directions, and may 
reach the earth's surface, where they are felt over a larger 
or smaller area as an earthquake. 

The subterranean movements which cause great earthquakes are 
generally sufficient in amount to cause perceptible displacement of 

(264) 



EARTHQUAKES. 265 

the surface strata. Such displacements are generally of a few 
inches or a few feet only, and usually consist of the elevation or 
depression of the strata on one side of a line of fault. Often the 
movement causing an earthquake occurs beneath the sea, and the 
overlying water conceals the surface displacement. Some severe 
earthquakes, and the great majority of minor ones, are caused, 
however, by subterranean fractures or movements so small that no 
sensible alteration in the surface topography results, the movement 
being entirely taken up by the redistribution of internal stresses. 

Earthquakes are very common ; it is probable that 
there is one every hour of the day in some part of the 
earth. They are more frequent in some regions than in 
others, but there is no region where they may not occa- 
sionally be felt. In mountain regions, and especially in 
the highest and youngest mountains, erosion is most rapid, 
and on the sea bottom, along the margins of continents, 
sedimentation is greatest ; in these regions, therefore, sub- 
terranean pressure and temperature changes are most rapid, 
and earthquakes are frequent. Earthquakes are most 
frequent along the convex or Pacific side of the great 
continental plateau, which is bordered by the highest and 
youngest mountains. Earthquakes are least frequent in 
comparatively low and level inland regions, as the central 
part of South America, central United States and British 
America, Russia, Siberia, and central Australia, and in 
the sea bottom far from land. In these regions respect- 
ively, erosion and sedimentation are very slight, and occa- 
sion a relatively slow accumulation of subterranean stresses. 

On an average, thirty or more earthquakes occur in the United 
States annually. More than 500 were recorded in this country dur- 
ing the sixteen years between 1872 and 1887, and doubtless many 
others occurred which were not recorded, especially in the sparsely 
settled region west of the Rocky Mountains. These earthquakes 
were distributed thus : 

West of Rocky Mountains . . 240 ; average, 1 every 24 days. 
East of Appalachian Mountains, 210; " " " 28 " 
Mississippi Valley 80; " " " 73 " 



2 66 PHYSICAL GEOGRAPHY. 

The regions shaken by the most perfectly recorded of these 
earthquakes are approximately indicated on the accompanying map. 
It gives a graphic idea of how common earthquakes are even in the 
Mississippi Valley, which is one of the most stable regions on the 
land surface of the globe. Many earthquakes west of the Rocky 
Mountains probably affected a more extensive region than some of 
those indicated, but are not shown upon the map because their 
record in that thinly settled region does not indicate even approxi- 
mately their extent. 

Elastic Waves. — Almost all rocks, and many other 
solids, are highly elastic within minute limits ; that is, they 
yield very slightly under great stress or pressure, and re- 
gain their former shape or volume immediately when sud- 
denly relieved from stress. It is this property which 
causes an ivory or a glass ball to rebound when dropped 
upon a hard surface. In consequence of the elasticity of 
rock, the sudden relief from stress afforded by the occa- 
sional movements of subterranean strata throws the adja- 
cent rock molecules into a state of very slight vibration. 
While the distance through which the molecules vibrate 
is so slight as to be invisible, the energy of the vibra- 
tion may be very great — nearly as great as that of the 
accumulated stress which caused the movement. The 
vibrating molecules communicate a similar but less ener- 
getic vibration to neighboring molecules, and these to 
molecules still more distant. In this way a thrill or 
tremor, called an elastic wave, is transmitted through the 
rock from the locality of the initial jar in all directions 
with wonderful rapidity but gradually decreasing energy. 
Upon arriving at the earth's surface, the energy of 
the invisible vibration of molecules in the elastic wave 
causes the visible movements of the surface soil or the 
sensible shocks which constitute an earthquake. Hence, 
there are three features to be considered in regard to an 
earthquake: (i) the origin of the jar; (2) the transmission 




(267) 



268 PHYSICAL GEOGRAPHY. 

of its energy to the earth's surface, and (3) the effects of 
this energy upon the surface. 

The transmission of energy through a solid by an elastic wave 
may be made manifest by placing some light object, as a toy 
marble, in contact with one end of a long, heavy iron bar, and 
striking the other end of the bar with a hammer. The blow may 
not cause the slightest movement of the heavy bar as a whole, yet 
the molecules with which the hammer comes in contact are thrown 
into invisible vibration. The vibration is transmitted from molecule 
to molecule through the bar, and may impart sufficient energy to 
the light marble to cause it to start visibly and perhaps violently 
forward. If, instead of a marble, a cake of moist clay be made to 
adhere to the end of the bar, the transmitted energy may be suffi- 
cient to detach the clay cake. These experiments illustrate per- 
fectly the three features of an earthquake : the hammer blow 
represents the initial jar ; the invisible molecular vibration propa- 
gated through the bar represents the invisible transmission of 
energy as an elastic wave through the heavy crust of the earth ; 
and the visible movement of the marble or clay represents the 
effect of this wave upon comparatively light surface objects, such 
as the soil, buildings, or weakly attached masses of cliffs, etc. 

The Origin of the Jar. — All knowledge respecting the 
deeply buried origin of a jar must be gathered from ob- 
servations of the effects of the earthquake at the earth's 
surface; and many circumstances render it exceedingly 
difficult to draw proper inferences from these observations, 
which are themselves difficult to make with accuracy. It 
seems to be true, however, (1) that the origin is seldom 
more than 1 2 miles below the surface ; it may occur, 
however, at any depth less than this ; (2) that the size of 
the shaken region bears a certain relation to the depth of 
the origin, a small shaken region always indicating a rela- 
tively shallow origin ; (3) that the energy of the jar is 
approximately indicated by the size of the shaken region, 
a large shaken region indicating a great accumulation of 
energy or stress in the initial jar; (4) that the origin is 



EARTHQUAKES. 269 

seldom a point, but generally a line or narrow district, 
which may be many miles in length; and (5) that the 
subterranean stresses are not relieved by a single move- 
ment of the strata, but rather by a quick succession of 
movements, causing a series of jars, and it is such a series 
that causes an earthquake. The series lasts from a second 
or two to several minutes. The jars of a series quickly 
increase to a maximum of energy, and then more gradu- 
ally become less energetic. 

The redistribution of internal stresses following the movements 
of the strata which cause a great earthquake, generally results in 
lesser movements of other subterranean strata in the neighborhood, 
and thus originates minor earthquakes, which may affect the same 
region at irregular intervals for a year or more after the occurrence 
of the great earthquake. 

The Transmission of Earthquake Shocks. — An 

elastic wave travels through rocks with wonderful rapidity; 
still its transmission occupies a certain amount of time. 
Hence, an earthquake shakes places which are near to the 
origin sooner than places successively more distant. The 
part of the earth's surface which is nearest to the" site of 
the subterranean jar lies directly over it. Therefore, in 
any earthquake the district in which the shocks occur 
earliest is called its epicentrum (on the center). This is 
located at some approximately central part of the shaken 
region. At the boundary of the shaken region the earth- 
quake occurs some seconds or minutes after the epicen- 
trum is shaken. 

The velocity of elastic waves is greater in compact 
solids than in those of looser texture. It is about four 
miles a second in steel, about two miles a second in com- 
pact granite, and but 800 to 1,000 feet a second in compact 
sand or clay. The different strata near the earth's surface 
vary greatly in compactness, but as the depth increases, 



270 



PHYSICAL GEOGRAPHY. 



the weight of overlying rocks compacts all the strata, but 
especially the more porous and compressible, until, at the 
comparatively slight depth of a few thousand feet, all the 
strata attain a great and nearly uniform degree of com- 
pactness. Hence, the earth's crust may be divided into 
two layers or shells: (1) a comparatively thin outer shell 
of exceedingly various density, through which elastic 
waves travel at greatly differing velocities; and (2) a thick 
inner shell of great and approximately uniform compact- 
ness, through which the waves travel with a nearly uni- 
form velocity, which is thought to exceed three miles a 
second. 




Fig. 112. 

The diagram (Fig. 112) represents a section of the earth's crust, 
SF the surface, and DL the division (say at the depth of 1 mile) 
between the two shells. Suppose the Outer shell to the left of C is 
more compact than to the right of C, and the inner shell is still 
more compact. Let O, at a depth of 5 or 6 miles, be the origin of 
an earthquake, and the spaces between adjacent curved lines the 
distance traveled by the elastic wave in one second of time. As 
the wave spreads and enlarges, it maintains a general spherical or 
spheroidal shape in the inner shell, but becomes flattened and other- 
wise deformed as it passes through the outer shell of varying com- 
pactness. In nature, the shape of the wave is much more irregular 
than represented, especially in the outer shell ; and it is on account 
of these great irregularities of shape, speed, and effect resulting 
from the passage of the wave through the superficial strata, that it 
is so difficult to discover the laws of earthquakes from surface ob- 
servations. The diagram indicates that the wave reaches the sur- 



EARTHQUAKES. 27 1 

face first at the epicentrum E ; that it then requires one second to 
spread to J-fand V; two seconds for it to reach X and Y, while the 
wave does not shake 6" and F until four seconds after E is shaken. 

Energy. — Not only does the earthquake occur earliest 
at the epicentrum, but its energy is greatest in this lo- 
cality. The energy is least at the boundary of the shaken 
area. The diagram (Fig. 1 1 2) renders this evident. The 
energy which causes the disturbance in every part of the 
shaken region was originally concentrated at O. It spreads 
from in the elastic wave. As the wave enlarges, the 
energy is distributed over its increasing circumference, and 
the amount at any one point in the wave constantly 
diminishes ; hence, the point E in the small circle 2, 2, 
receives more energy than the points X and Y in the 
larger circle 4, 4, and much more than the points 5 and 
F in the still larger circle 6, 6. 

The distance to which an elastic wave is propagated 
depends (1) on the amount of energy at the origin, and 
(2) upon the compactness and uniformity of the strata. 
A jar occurring in the outer shell might, on account of 
its nearness to the surface, cause an exceedingly violent 
earthquake at and about the epicentrum, but on account 
of the rapid dissipation of energy in passing through 
strata of loose texture, the earthquake would probably 
affect but a small surface area. If the same amount of 
energy should cause a jar at a much greater depth, the 
epicentrum, being farther from the origin, would be less 
energetically shaken, but the elastic waves would spread 
faster and farther through the deep, compact strata, and 
might carry to considerable distances enough energy to 
penetrate the thin outer shell, and thus cause the shaking 
of a much more extensive surface region. It seems prob- 
able, however, that most jars in the inner shell are more 
energetic than those which occur in the outer shell, for it 



272 PHYSICAL GEOGRAPHY. 

must, in general, require a greater accumulation of energy 
to cause movement in deeply buried strata than in those 
pressed upon by a less weight of overlying rocks. 

The subterranean explosions and the fracturing and Assuring of 
the strata, which frequently accompany volcanic eruptions, are often 
sufficiently energetic to cause violent earthquakes in the immediate 
vicinity of the volcano ; but such earthquakes never affect a large 
region because the origin is at a comparatively slight depth. 

Surface Effects of Earthquakes. — The vibration of 
rock molecules which constitutes an elastic wave consists 
chiefly of a minute forward movement in the direction the 
wave advances, and a minute backward movement toward 
the origin. Consequently, the earthquake at the epicen- 
trum consists of an up-and-down shaking, while at other 



o 

S.F. = Earths surface E.= Epicentrum 

O. = Origin V. X. & Y. = Points successiuely more distant 

Double headed arrows = direction of vibration of rock molecules 

Fig. 113. 

places in the shaken region the movement becomes more 
and more horizontal as the distance from the epicentrum 
increases. This is made plain by Fig. 113, which also 
indicates that the epicentral area s-v, in which the princi- 
pal movement is up and down, forms a very small part 
of the whole shaken area, its size increasing with the depth 
of the origin. 

The boundary of this epicentral district can sometimes be located 
on the ground with considerable accuracy as inclosing the area 
where the relative violence of the earthquake has manifestly been 
greatest. When this can be done, it affords the best known method 
for calculating the depth of the origin. 

Within the epicentral district the earthquake tends 
to throw surface objects upward. Men and heavy masses 
of rock have been thrown into the air, and large trees 



EARTHQUAKES. 273 

have been uprooted and thrown upward. Foundations 
of brick or stone masonry under buildings in the epicen- 
tral district are sometimes actually crushed by the sud- 
denness of the upthrust when the enormous energy of the 
elastic wave arrives at the surface beneath them, just as a 
sudden upward blow on a suspended mass of wax may 
crush and indent it, while if the same amount of energy 
had been applied more gradually, it would have simply 
moved the whole mass of wax without indenting it. 

Without the epicentral district, the principal im- 
pulse of the earthquake is more nearly horizontal. Still 
there is some up-and-down movement, and this may im- 
part a slight but yet sensible motion to the comparatively 
light surface strata, just as sensible motion was imparted 
to the clay cake on the end of the iron bar. This slight 
up-and-down movement imparted to adjacent parts of the 
ground in quick succession, as the earthquake spreads 
rapidly outward, throws the surface into an actual undu- 
lation or wave, similar to a water wave that spreads out- 
ward from an agitated point in the surface of a pond. 

Cracks and Fissures in the Soil. — The passage of 
the crest of such an earth-wave or undulation often causes 
fissures many feet deep to open in the soil, which some- 
times remain open, but more frequently open and close 
alternately as the crest and trough of successive undula- 
tions pass under them. The conduits of subterranean 
water are generally disarranged by such fissures, and thus 
the location of surface springs is frequently changed, tem- 
porarily or permanently, by an earthquake, while the un- 
derground pressures of the passing undulations often cause 
the ejection from the fissures of water, sand, and mud to 
a height of many feet. 

Buildings are caused to rock or sway back and forth 
by the passage of such undulations, the oscillation being 
p. G.-16. 



2 74 PHYSICAL GEOGRAPHY. 

greater in the upper part of the building than below, just 
as the part of a ship which oscillates through the greatest 
distance when a wave passes under the vessel, is the top 
of its masts. A very slight movement of this kind is 
sufficient to crack the walls of rigid buildings, and to oc- 
casion a swing at the top of high houses great enough 
to cause the downfall of chimneys or even of the walls 
themselves. Well-made frame buildings, on account of 
the greater play which they allow at the places where the 
various timbers are joined together, are not so apt to be 
destroyed by earthquakes as rigid brick or stone houses. 
It is by this quiet oscillation of buildings that many ex- 
tensive earthquakes are recognized over by far the greater 
part of the shaken area, most or all of the "shocks" be- 
coming so slight in the transmission to great distances 
that they are scarcely perceptible. 

The surface violence of earthquakes varies greatly in closely ad- 
jacent localities, owing to the differences in texture of the superficial 
strata. The localities where earthquakes are apt to be least violent 
are those situated near the center of an extensive region underlaid 
to a great depth with strata of loose texture, for all but the most 
energetic waves are quenched in this loose material before reach- 
ing the surface ; but if the depth of the loose material is only 
slight, the locality is apt to be more, violently shaken than one on 
compact rock, for the elastic wave may impart to a thin and com- 
paratively light layer of loose material at the surface, as it did to 
the clay cake on the iron bar, a sensible motion, which is apt to be 
sufficient to destroy the most substantial buildings. 

Deep sounds or rumblings frequently accompany or 
follow earthquakes, especially in and about the epicentral 
district. They are caused by vibrations imparted to the 
air by such of the rock vibrations as are of the proper 
length and rapidity to excite in us the sensation of sound. 
The air transmits these vibrations to the ear precisely as 
it does those of the string of a violin, and the air vibra- 
tions become sensible as sound in both cases. 




(=75) 



276 PHYSICAL GEOGRAPHY. 

Sea Waves Caused by Earthquakes. — When the 
epicentrum of an earthquake occurs beneath the sea, the 
upward impulse of the sea bottom may upheave the over- 
lying water and cause a series of sea waves, which spread 
in all directions to great distances with a velocity which 
increases with the depth of the water, but which, even in 
the deepest ocean, is not nearly so great as the velocity 
of an elastic wave through a compact solid. Hence, if 
the earthquake which causes the sea wave is felt at all on 
land, it is felt some time before the arrival of the sea 
wave. 

In the deep open ocean these sea waves are so long and so low- 
that their passage beneath a vessel is generally imperceptible; but in 
entering shoal water, as land is approached, the waves become 
shorter and higher, after the manner of the tide waves, and their 
arrival at the shore is indicated by the rapid rise of the water above 
its usual level. Such a rise of 50 to 100, or even 200 feet, has been 
known. The greatest waves are produced by an earthquake caus- 
ing a very energetic disturbance in an epicentral district located not 
very far from the coast, and yet beneath deep water. These con- 
ditions are most likely to occur on the steeply sloping convex margin 
of the continental plateau, and hence great sea waves are more 
frequent on the abrupt Pacific coasts than on the more gently slop- 
ing Atlantic shores, though great waves inundated the steep coast 
of Portugal after the great Lisbon earthquake. 

Among the earthquakes which have occurred in the 
United States, four are specially prominent on account of 
the great area over which they were felt. 

In 181 1 an earthquake shook the entire territory between western 
Texas and Washington City, and the Gulf of Mexico and the Great 
Lakes, an area of more than a million square miles. It was caused 
by subterranean movements which occasioned the settling to a 
depth of 15 or 20 feet of a large district about New Madrid, Mo., 
below the junction of the Ohio and Mississippi rivers. Portions of 
the sunken district, 20 miles or more in length, were afterward 
flooded by the river, and became Reelfoot Lake in north-western 
Tennessee, and Big Lake between Missouri and Arkansas, 






EARTHQUAKES. 277 

In 1872 an earthquake was felt over the Pacific slope from Oregon 
far into Mexico, and from the coast eastward to Utah and New 
Mexico. The surface indication of the subterranean movements 
which caused this earthquake was the tilting and shifting of a great 
block of the earth's crust 40 miles long and one fourth of a mile 
wide in Owens Valley, CaL, at the east base of the Sierra Nevada. 
This block settled about 25 feet along its western side, and about 5 
feet along its eastern side. Many houses in the town of Inyo, near 
the epicentral district, were destroyed, and several lives were lost. 

In 1886 an earthquake occurred which shook the region from 
Wisconsin to Cuba and the Bermuda Islands, and from Maine to 
the mouth of the Mississippi, an area of nearly 3,000,000 square 
miles. Its epicentrum was about 15 miles north-west of Charleston, 
S. C. Few known earthquakes anywhere have shaken a larger 
area, and hence the jar which caused the Charleston earthquake 
must have been among the most energetic of which the world has 
record ; and yet many earthquakes have been much more violent. 
Hence its origin must have been one of the most deeply buried. 
The boundary of its epicentral district was well marked, and from 
it the depth of the origin was calculated to be about 12 miles. 
Within the epicentral district was the little collection of frame build- 
ings, called Summerville. This was terribly shaken, and a dozen or 
more of its wooden houses were wrecked. In Charleston almost all 
the brick buildings were severely injured, and a large number com- 
pletely wrecked. Many chimneys were overthrown as far distant as 
Atlanta, Ga., (250 miles), Asheville, N. C, (230 miles), and Raleigh, 
N. C, (215 miles). Had the epicentrum occurred a few miles 
farther south-east, or had the city been underlaid by a less depth of 
loosely compacted strata, Charleston would probably have been laid 
in ruins, and the loss of life would have been vastly greater than it 
was. 

In May, 1887, an earthquake shook the region between the Colo- 
rado and the Rio Grande from Utah almost as far south as the city 
of Mexico. Its epicentral district, in the Mexican state of Sonora, 
included the town of Babispe, which was entirely destroyed. The 
epicentral district was found to be traversed by a new fault 35 miles 
long, of which the vertical displacement averaged 8 feet. 



CHAPTER XX. 

VOLCANOES. 

-Bow thy heavens, O Lord, and come down : touch the mountains, and they shall 
smoke. — Psalm cxliv : 5 

A volcano is essentially a collection of ducts or fissures 
in the earth, from which intensely hot gases and rocky 
material have been discharged. The rocky material dis- 
charged usually accumulates around the ducts into a more 
or less isolated and cone-shaped heap called a volcanic cone, 
which may reach an altitude of many thousand feet, and 
cover an area of hundreds or even thousands of square 
miles. The principal mouth, or vent, of a volcano usu- 
ally occurs in a hollow, called the crater, near the summit 
of the cone. 

Volcanic eruptions vary greatly in intensity at differ- 
ent times and places. A few volcanoes are constantly 
discharging matter ; usually, however, volcanic activity is 
intermittent, — eruptions lasting days, weeks, or months, 
alternating with dormant periods, lasting years or even 
centuries, during which there is no discharge. Continu- 
ous eruptions, or those recurring at short intervals, are 
seldom very violent ; violent eruptions generally succeed, 
and are followed by, proportionately long periods of rest. 
Eventually, after perhaps thousands of years of such con- 
stant or intermittent activity, the great heat beneath a 
vent subsides permanently, and the volcano becomes ex- 
tinct. 

(278) 



VOLCANOES. 279 

The materials discharged in eruptions are chiefly 
melted rock or lava and steam. When the lava rises in the 
ducts, its entire mass seems to be permeated with steam, 
which escapes from it more or less explosively. Rela- 
tively small quantities of other gases are generated by the 
heat from various minerals in the lava, and, by their action 
upon each other and the surface rocks, frequently cause 
deposits of sulphur, alum, gypsum, salt, and other sub- 
stances to accumulate about the volcanic vent. Some of 
these gases are combustible, and are ignited by the heat ; 
but the flames are only feebly luminous, and never form 
a conspicuous feature of an eruption. 

The lava is discharged both in streams and in frag- 
ments, the proportion discharged in either manner de- 
pending largely upon the violence of the eruption. In 
very quiet eruptions, the lava is discharged chiefly in 
streams, while in some very violent eruptions it is entirely 
ejected in fragments ; usually, however, lava is ejected in 
both ways during an eruption. 

The violence of an eruption depends to a great ex- 
tent upon the fluidity of the lava and the abundance of 
its permeating steam. With stiff lava in the ducts, the 
steam, when abundant, escapes spasmodically with terrific 
explosions, hurling to prodigious heights and distances 
vast quantities of glowing lava masses, and blocks of rock 
torn from the crater or the sides of the duct. The lava 
masses are of various shapes and sizes, and, cooling in 
the air, fall as globular bombs; jagged and slag-like 
cinders or sconce ; glassy and bubble-impregnated pumice; 
gravelly lapelli; sand; and the fine, glassy dust called 
volcanic ashes. Deluges of rain from the condensing 
steam falling on the cone transform the dust into a fine 
fluid mud, which hardens into a compact rock called tuff, 
while the larger fragments are cemented together into 



28o PHYSICAL GEOGRAPHY. 

volcanic conglomerate. The steam escapes more readily 
and continuously from very fluid lava ; hence, a violent 
eruption of such lava seldom occurs. But even very fluid 
lava is viscous, like syrup, and the escaping vapors carry 
up from its surface long filaments, which, when cool, re- 
semble spun glass. This is called Pele's Hair in Hawaii, 
where it is formed in great quantities. 

The fluidity of lava depends largely upon its mineral composi- 
tion. When composed largely of infusible silica it is called trachytic 
lava, which is never thoroughly melted, and is always stiff. When 
less silica is present, it is called basaltic lava. This- melts more 
readily, becoming as fluid as melted glass, and is apt to resemble 
glass upon cooling rapidly. 

Lava streams issue at a white heat either over the 
edge of the crater, or more frequently from fissures in the 
side of the cone, and flow rapidly at first. Very soon 
a cool, solid crust forms over a stream, which moves very 
slowly, while the interior, prevented by the non-conduct- 
ing crust from cooling quickly, flows faster, and constantly 
bursts through the cool crust that rapidly forms on the 
front of the stream. The flow of the interior sometimes 
leaves long hollows or tunnels beneath the crust, but usu- 
ally the slow advance and contraction of the cooling crust 
break it up into countless blocks which settle down into 
the cavities beneath (Fig. 114). 

The crust of a lava stream is such a poor conductor that the 
interior may remain at a red heat for many months after its eruption, 
and during this time the whole mass may be imperceptibly advanc- 
ing. Enormous volumes of steam escape through the crevices in 
the cool crust of a fresh lava stream from the hot interior, some- 
times throwing up miniature cones. The steaming vents on a lava 
stream are called spiracles or fumaroles. The length of lava streams 
depends chiefly upon the amount of lava erupted and its liquidity. 
Streams from 1 to 5 miles long are very common, but the enormous 
outflows of very liquid lava in Hawaii and Iceland have reached 
distances of from 30 to 50 miles. 



VOLCANOES. 



28l 









e:Wi 






i^sslp 









Fig. 114. — An old Lava Stream on Vesuvius. 

General Shape of Cones. — The material ejected is 
deposited over a wide area, but in decreasing quantities as 
the distance from the vent increases ; thus, the conical 
shaped heap or mountain is gradually built up by the de- 
posits of successive eruptions. The steepness of volcanic 
cones varies greatly, and depends partly upon the average 
liquidity of the lava in its various eruptions. Cones built 
chiefly of fragmental material are apt to be steep, since 
this material will stand at a slope of about 35°. Outflows 
of stiff lava also produce steep slopes. Very fluent lava, 
on the contrary, is apt to produce flatter cones ; those of 
Hawaii and Iceland have an inclination of less than io°, 
while in many parts of the world, as in the western part 
of the United States, the peninsula of India, the plateau 
of Abyssinia, and elsewhere, tens of thousands of square 
miles are covered many hundreds or even thousands of 
feet deep under successive outflows of ancient basaltic lava 
in practically horizontal layers. These lavas must have 



282 



PHYSICAL GEOGRAPHY. 



been very fluid at the time of their emission, since no cone 
at all seems to have been produced. It is even thought 
that they may have welled quietly up through numerous 
long fissures over the several regions without many of the 
phenomena characteristic of modern volcanoes. 

The internal structure of many cones has been laid 
bare by prolonged erosion after the volcano has become 
extinct. The fragmental deposits and lava streams of suc- 
cessive eruptions give the cone an irregularly stratified 
structure, the strata having a general dip away from the 
central ducts. These inclined beds are intersected by 



'/'■S\ TUFF & FRAGMENTS 
I I ) LAVA 
I ' I ' rH ROCK BENEATH CONE 




Fig. 115. —Ideal Section of a Volcano. 

numerous more or less vertical dikes, which radiate in 
all directions from the central ducts, and which are simply 
great lava-filled fissures which have been rent in the cone 
during eruptions. Some of these fissures reach to the 
surface of the cone, and such are the source of most lava 
streams ; others do not open through to the surface, and 
appear as dikes only after erosion has worn away the over- 
lying beds. A great fissure in a large cone is often 
marked by a line of minor or parasitic cones, thrown up 
successively along its course as it opens. The great cone 
of Etna has more than 200 parasitic cones, some of them 



p^jq ? Stratified rock 
^■H - Lava 

Ife MILES HIGH ,kloV ,V 



VOLCANOES. 

(,(■,_ erosion 

^o.wW" 



283 




Fig. 116. — Laccolite forming Mt. Hillers, Henry Mountains. 

over 600 feet high. In some cases, the rocky platform be- 
neath has subsided to a greater or less extent as the cone 
accumulated. In other cases, the ascent of stiff trachytic 
lava in the central ducts seems to have bent the adjacent 
strata upward, and very frequently it is found to have 
penetrated horizontally between strata, forming deeply 
buried horizontal sheets of great extent. 

These intrusions of trachytic lava between deeply buried strata 
are not only very extensive but sometimes very thick, when they 
are called laccolites. Their formation sometimes pushes up the 
overlying strata to form great dome-like hills or mountains at the 
earth's surface. The Henry Mountains, an isolated group in 
southern Utah, seem to have been upheaved by such a subterranean 
intrusion of lava, which, instead of reaching the surface as an ordi- 
nary volcano, spread out between the strata at a depth of between 
2 and 3 miles, forming 25 or more great circular laccolites or lava 
cakes, the largest of which is about 4 miles in diameter and 1% 
miles thick. The overlying strata, pushed up into domes by these 
laccolites, have been in places entirely removed by ages of subse- 
quent erosion, thus uncovering portions of the laccolites and reveal- 
ing the cause of the uplift (Fig. 116). In Colorado, New Mexico, 
and Arizona, as well as in foreign countries, trachytic lava in great 
masses has been partially uncovered by prolonged erosion ; many 
of these masses are doubtless laccolites, and indicate that such sub- 
terranean intrusions are by no means exceptional. 



284 PHYSICAL GEOGRAPHY. 

Changes in the Crater. — Every eruption changes the 
shape or size of a cone. Minor fragmental eruptions in- 
crease its height and bulk, but great or violent eruptions 
generally decrease its height, for the whole top of a cone 
may be shattered and blown off in fragments by the vio- 
lent explosions, or it may be engulfed when a copious dis- 
charge of lava drains away its subterranean liquid support. 
Thus, great eruptions often transform the upper part of a 
cone into a huge abyss, called a caldera, which may be 
several miles in diameter, and more or less completely sur- 
rounded by precipitous cliffs thousands of feet high. 
Minor eruptions build up a new cone within a caldera, 
which may fill and obliterate it before an eruption again 
occurs of sufficient energy to destroy the top of the cone. 

The crater of Kilauea (Hawaiian Islands) is a caldera 3 miles 
long and 2 miles wide. It almost always contains pools of liquid 
lava. The pools constantly overflow ; successive overflows, cooling, 
gradually build up the bottom of the pit, until suddenly, after an 
indefinite interval of years, subterranean fissures open, through 
which the lava pools drain away, and the bottom of the caldera 
sinks, while great slices often fall from the precipitous sides into the 
abyss and thus increase its area. Old craters and calderas often 
become filled with water. Such lakes are common in all volcanic 
districts. Lake Taupo, in New Zealand, is thought to occupy one 
of the largest calderas in the world. The lake is 20 miles in diam- 
eter, and is surrounded by cliffs 1,000 feet high. Crater Lake, in 
the Cascade Mountains of Oregon, occupies another caldera 7^ 
miles long and 5 miles wide. The lake is 2,000 feet deep, and is 
completely encircled by cliffs 1,000 to 2,000 feet high. From its 
surface an extinct cinder cone 600 feet high rises as an island, bear- 
ing a perfect crater in its summit. 

Eruptions. — Violent eruptions are usually preceded by 
muffled noises and earth tremors or shocks, caused proba- 
bly by the fracturing of subterranean strata. Then follow 
explosions which occasion heavy local earthquakes. The 
crater breaks up, and solid blocks and glowing lava frag- 
ments are scattered far and wide, while the steam escaping 



VOLCANOES. 285 

at each explosion, rising rapidly and condensing, adds a 
great globular mass to the dust and cloud canopy forming 
above (page 40). This canopy reflects the glow of the 
liquid lava in the ducts, and, together with the rapid 
ascent of incandescent fragments, produces the illusion of 
brilliant tongues of flame issuing from the crater. The 
column rising from the crater often reaches a height of 
several miles, within which is generated electricity, mani- 
fested by incessant flashes of lightning and terrific peals 
of thunder. Rainbows and halos are produced by the 
play of light through the water globules of the condensing 
steam, while the violent local updraught in the atmos- 
phere generally occasions terrific winds in the district sur- 
rounding the volcano. With an outflow of lava an 
eruption subsides, though sand and dust continue for 
some time to be discharged to great heights and in such 
quantities as often to exclude all daylight from a great 
extent of the surrounding country. Eventually the dis- 
charge of all solid matter ceases, but steam and gases 
continue for a long period to rise from crevices in the 
cone and from the lava streams. Quiet eruptions may or 
may not occasion earthquakes, and may consist simply of 
the issue of steaming lava streams from the side of the 
cone. This is usually preceded by a rise of lava into the 
crater, and an increased discharge of steam. 

The enormous energy of volcanic action is most strikingly dis- 
played in the infrequent but very violent eruptions. Thus, in a 
single night of 181 5 the top was blown from Tomboro, in the Malay 
Archipelago, reducing its cone from a shapely peak 2 miles high to 
a mere stump, less than half as high, with a huge caldera in the 
top. The eruption of Krakatoa, in 1883, was another instance of 
excessively violent volcanic action.. Its explosions were audible for 
2,000 miles in all directions, or over ^th of the earth's surface, and 
a perceptible layer of the dust ejected fell at all places within 1,000 
miles of the volcano; while the finest dust and vapor, shot up 15 or 
16 miles high, were generally distributed over the globe, causing, 



286 PHYSICAL GEOGRAPHY. 

while still suspended in the atmosphere, the peculiar red sunsets 
noticed in all parts of the world for months after the eruption (p. 104). 
The volcanoes of Hawaii often exude lava streams which cover 100 
to 200 square miles to a depth of 100 feet or more; but they are dis- 
charged so quietly that the display of energy is not striking. Re- 
peated outflows of this kind, however, during untold ages, have 
built up a great flat cone 6 miles high from the ocean floor, to form 
the lofty island which is half as large as New Jersey. This cone 
must contain material enough to cover the whole United States 50 
feet deep, and the energy required to heap it up is probably as great 
in the aggregate as that displayed during the life of any violently 
active volcano in the world. 

Gradual Decay of Volcanic Activity. — The cool- 
ing of the earth's crust beneath an old volcanic region 
is an exceedingly slow process. For ages after all other 
signs of activity have ceased, steam and volcanic gases 
continue to escape at some volcanoes from the numerous 
fissures. A volcano in this condition is said to be in the 
solfatara stage. Gradually, the heat in the superficial 
parts of the crust subsides until no longer great enough to 
convert all of the percolating water into steam, and the 
old volcanic region becomes a district of hot springs. 

Most of the warm springs in the world, and nearly all the very 
hot ones, occur in or near volcanic formations, though frequently in 
localities where no volcanic eruption has taken place for hundreds, 
and probably for many thousands of years. 

Geysers. — The hot springs of volcanic regions are 
characterized not only by their high temperature, but by 
the immense quantities of mineral matter, usually silica, 
which they bring to Ae surface and deposit over their 
neighborhood in fantastic and intricate forms. Often the 
deposit forms extensive terraces of silicious sinter through 
which the streams rise into deep, funnel-like basins. If 
the water enters such a basin slightly above its boiling 
temperature, the spring may become a geyser (spouter or 
gusher). ' 



VOLCANOES. 287 

The water near the surface, chilled by the air, is kept beneath its 
boiling temperature, while the water below is kept from boiling by 
the pressure of that above. Thus, the lower water becomes super- 
heated, and gradually heats the surface water, which at last begins 
to boil. This relieves the pressure on the water immediately below, 
which, being above its boiling point, vaporizes explosively, and 
forces into the air a cloud of steam and a jet of the overlying water. 
This considerable relief from pressure is followed by louder explo- 
sions in the still hotter water beneath, and the more violent dis- 
charge of water jets and steam clouds into the air. The explosions 
and discharges continue until the basin is emptied and the water in 
the conduits is chilled below its boiling point by exposure to the air. 
The eruption then ceases, and the water rises quietly in the basin 
until the conditions are suitable for another eruption. The eruptions 
of a geyser occur at more or less regular intervals of time, but these 
intervals vary in different geysers, from a few minutes to many 
hours or days. By the continued mineral deposit, the shape and 
dimensions of a basin may be so changed as to convert a hot 
spring into a geyser, or a geyser into an ordinary hot spring. 

Geysers occur in many volcanic districts over the world. They 
are most numerous and powerful at the sources of the Missouri in 
Yellowstone Park, Wyoming ; near Mount Hecla, in Iceland ; and 
in the North Island of New Zealand ; but are also found in Mexico, 
the West Indies, the Azores, Thibet, the Malay Archipelago, the Fiji 
Islands, and possibly other places. The so-called geysers of Cali- 
fornia and Nevada are violently boiling springs rather than true 
periodic geysers, though they are closely associated phenomena. 
In Yellowstone Park there are more than 3,000 hot or boiling springs, 
including 71 geysers, of which the most noted are: the Giantess, 
which throws jets 250 feet high, at intervals of several weeks; the 
Bee Hive, spouting 219 feet, at intervals of 14 to 16 hours; Grand 
Geyser, 200 feet, at intervals of 16 to 30 hours ; the Giant and Castle 
geysers, spouting about 200 feet high, and Old Faithful (see frontis- 
piece), which every hour throws up jets to a height of about 150 
feet. Whenever hot springs occur in clay formations, the water in 
the basin is apt to become more or less muddy from repeated caving 
in of the banks. Sometimes the pool thus acquires a thick, por- 
ridge-like consistency, and if the temperature be high enough to 
cause the water to boil, the explosion of steam-bubbles beneath the 
surface scatters the mud about. Such mud springs are called mud 
vo/cajioes. They are common in all hot-spring districts. 



288 PHYSICAL GEOGRAPHY. 

Distribution. — The indications of past or present vol- 
canic action are found in all latitudes and longitudes, and 
at all elevations. They occur on the continents, and on 
both the continental and oceanic islands, while several re- 
corded submarine eruptions attest their occurrence upon 
the sea bottom. Volcanic activity seems to have been 
present somewhere on the earth's surface throughout geo- 
logical time, but has gradually shifted the site of its 
activity to new areas during the long course of the world's 
history. The total number of localities in the world in 
which are found indications of volcanic action, ancient or 
modern, would reach tens of thousands. About 300 vol- 
canoes are known to be active. About one half of these 
occur on the continental islands lying south-east and east 
of Asia, and extending from New Zealand to Alaska. 
Within this region, volcanism is at present more energetic 
than elsewhere on the globe. About one fourth of the 
active volcanoes are distributed irregularly along the ele- 
vated western margin of the American main-land, from 
Alaska to the Str. of Magellan. There are about 45 
active vents in North and Central America, and about 37 
along the Andes. A few have been discovered in east 
Africa, and on islands along that coast. Thus, fully three 
fourths of all active volcanoes known lie just within the 
convex or generally rising border of the continental 
plateau. Only one eighth of the world's active volcanoes 
occur elsewhere on the continental plateau, and these are 
found in widely separated groups. Three of these groups — 
Iceland, with 1 3 active vents ; the Lesser Antilles, with 6, 
and the Canary Islands, with 3 — occur in rising localities 
on the generally subsiding concave margin of the plateau ; 
a fourth group of 7 vents occurs in a rising area on the 
margin of the deep Mediterranean depression, while the 
rest, 5 or 6 in number, are found along the northern 







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290 



PHYSICAL GEOGRAPHY. 



margin of the geologically recent highlands of Asia. The 
remaining eighth of all the active volcanoes are distributed 
along the great submarine ridges which traverse the 
oceanic depressions. There are about 20 active vents in 
the depression of the Pacific, 10 in that of the Atlantic, 
and 2 or 3 in that of the Indian Ocean, while there are at 
least 2 within the Antarctic Circle. 




Fig. 117. — Volcanic Necks in western New Mexico. 



Indications of former volcanic action which is now either 
dormant or extinct are also found in all the regions mentioned 
above, and in many other localities over the land areas, chiefly in 
highly tilted and disturbed strata, such as are frequent in mountain 
regions ; in fact, almost every mountain range in the world has 
associated with it, either in its mass or near its base, vestiges of 
volcanic action. It is generally true that indications of very re- 
cently extinct action are more numerous toward the convex side 
of the continental plateau, while vestiges of very ancient and long 
extinct volcanism are more numerous on the concave side. Indica- 
tions of very recent action are found throughout the West, and of 
very ancient volcanic action throughout the eastern part of the 
Union. In the Cascade Range are many great volcanic cones be- 
tween 10,000 and 14,000 feet high. Some of them are still emitting 



VOLCANOES. 291 

steam and gases. At Feather Lake, in northern California, a fresh 
lava stream 3% miles long and a mile wide occurs, and is said to 
have been erupted in 1850. Fresh lavas, possibly a century or two 
old, are found in Utah and Arizona. In western New Mexico are 
lava streams 24 miles long and 4 miles wide, which can not be 
many centuries old ; but in the same neighborhood are the remains 
of much older lava streams, which in the tertiary era flooded this 
region from many vents. Prolonged erosion has completely re- 
moved most of this old lava cap, and also a great thickness of the 
strata beneath it, leaving, however, on the site of each vent an 
isolated hill or mountain composed of the hard lava which solidified 
in the duct when the volcanism subsided. Scores of such " volcanic 
necks," from 800 to 1,500 feet high, are found in that vicinity. 
(Fig. 117.) Indications of still older volcanic action are found in 
the tilted lava sheets which traverse the eastern part of the Union 
from Maine to South Carolina. The Palisades of the Hudson, and 
Mt. Tom and Mt. Holyoke, of Massachusetts, are such sheets. 
These sheets were erupted early in the mesozoic era, long before the 
Rocky Mountains were upheaved. The tilted lava sheets which 
form Keweenaw Point and the Gogebic Range south of Lake 
Superior, are still older. The eruption of these sheets took place 
before the Alleghanies were upheaved — in early paleozoic times. 

Causes of Volcanic Action. — All active volcanoes 
seem to occur in regions which are rising. It is probable 
that' the heat which melts subterranean rock masses into 
lava, and leads to its ejection, is but a peculiar manifesta- 
tion of the same energy which causes the upheavals of the 
earth's crust. Whatever be the causes of these move- 
ments, it seems certain that the friction of the moving 
rock particles against each other would generate excep- 
tionally intense heat at certain places within the rising 
mass. The heat in these localities may become great 
enough to liquefy the more fusible rocks at a compara- 
tively slight depth, though not great enough to liquefy the 
less fusible surrounding rocks. Thus, a subterranean 
cavity, or vesicle, full of molten lava, is formed, which 
may be many miles in horizontal dimensions, and many 

hundred feet in vertical depth. When, owing to the pe- 
p. G.-17. 



292 PHYSICAL GEOGRAPHY. 

culiarities in mineral composition which determined its 
fusibility, the molten mass is lighter, bulk for bulk, than 
the solid rocks above, the great weight of the latter causes 
them to sink down into the cavity, squeezing the molten 
lava upward into the fissures caused by the subsidence. 
If the difference in weight between the lava and the solid 
mass above is but slight, the lava may rise only part way 
to the surface, and spread out between the strata to form 
subterranean lava sheets or laccolites ; but if the difference 
in weight is great enough, the lava is squeezed upward to 
the surface, to overflow and form a volcano. 

Thus, steam is probably not an essential factor in bringing the 
lava up to the earth's surface, though all lava seems to be perme- 
ated with steam when it "reaches the surface, and the degree of vio- 
lence of volcanic eruptions probably depends upon the manner in 
which this steam escapes from lavas of different mineral composi- 
tion, and from the same lava at different temperatures and pressures. 
It seems probable that the water percolating through all rocks is 
converted at some depth into steam by the subterranean heat, and 
as such is absorbed or occluded by the molten lava in very much 
greater quantity than the lava is able to retain when its temperature 
and pressure diminish as it rises toward the earth's surface. Hence, 
the excess of the absorbed steam escapes, or is excluded, more or 
less explosively, according to the viscosity of the lava, producing a 
more or less violent eruption. It is a somewhat similar exclusion 
of carbonic acid gas, absorbed by water under high pressure, that 
produces the effervescence of soda water when the pressure within 
the "fountain" is relieved, by opening the nozzle. 



PART V. — WEATHER AND CLIMATE. 



CHAPTER XXL 

WEATHER AND CLIMATE. 



When it is evening; ye say, It will be fair -weather: for the sky is red. And 
in the morning, It will be foul weather to day : for the sky is red and lowering. — 
Matthew xvi : 2, 3. 

Weather is the condition of the atmosphere at any 
time and place with respect chiefly to its temperature, 
humidity, clearness or cloudiness, rain, fog, or snow, and 
wind. 

Changes of Weather. — The weather is every-where 
constantly changing, owing to the diurnal and seasonal 
variations of temperature. But, in addition to these com- 
paratively regular changes, others, much less regular, take 
place as a result of the passage of cyclonic winds or 
storms, which may quickly replace the air over any 
locality with other air having a very different temperature 
and humidity. 

In the torrid zone cyclones seldom occur, excepting 
in the western part of the tropical oceans; and hence the 
weather-changes in that zone, depending principally upon 
the variations in the position of the sun, occur with great 
regularity, the same changes often taking place at the 
same hour, day after day, for weeks together, ever}' year. 

(293) 



294 PHYSICAL GEOGRAPHY. 

In temperate zones, cyclonic winds are much more 
common. Between latitudes 40 and 70 , where they are 
of most frequent occurrence, an endless procession of 
cyclones and anticyclones moves eastward. Though their 
general movement is easterly, the different whirls seldom 
move in exactly the same direction (see chart, page 91) 
or at the same rate of speed ; hence, different ones pass 
over the same locality at irregular intervals. Each whirl 
produces variations of weather as it passes over a locality, 
which modify in a marked degree those regular variations 
due to the alterations in the relative position of the sun; 
and as the whirls arrive at irregular intervals, the weather- 
changes are as irregular in temperate latitudes as they are 
regular in equatorial regions. 

"Weather Probabilities. — From long observation of 
the paths traveled by cyclones and anticyclones under 
different circumstances, the officers of the United States 
Weather Bureau are enabled to estimate with some accu- 
racy the course which any cyclone or anticyclone observed 
in or near the United States, will pursue during the en- 
suing 24 or 36 hours ; and it is upon this estimate that 
the weather predictions are based which the Weather 
Bureau furnishes for publication throughout the Union 
every morning. 

The use of the telegraph in weather prediction began with its 
extension over this country in 1844-48, but was first systematically 
done by Prof. Joseph Henry and Prof. J. P. Espy about 1850. It 
was begun in Europe in 1854, and after the war was revived by the 
Cincinnati Chamber of Commerce for mercantile purposes. From 
this followed the action of Congress authorizing storm and flood 
predictions to be made at first by the Signal Service of the Army, but 
at present by the Weather Bureau. The atmospheric pressure and 
the condition of the weather are carefully observed twice a day at the 
same moment of time in all parts of the country, and the results are 
telegraphed to the central office at Washington. Here the data are 
entered upon a map, the isobars drawn, and the successive positions 



WEATHER AND CLIMATE. 295 

of cyclones and anticyclones, as they travel over the country, thus 
indicated. It has been found that the topography of the country, 
the sunshine, and the relative temperature and moisture in adja- 
cent cyclones and anticyclones, modify the direction and speed 
of movement of each; but that in general the centers of cyclones 
move north-eastward over the United States, while anticyclones 
move south-eastward. Owing, however, to the ever-changing con- 
ditions in adjacent whirls, it is usually impossible to predict with 
any degree of accuracy, the course of any observed cyclone or 
anticyclone more than 24 or 36 hours in advance. 

Since the -wind whirls about the center of all 
cyclones in the same hemisphere in the same direction, 
the weather on corresponding sides of all cyclones is very 
similar, and the same is true of all anticyclones. Thus, in 
the northern hemisphere the winds in the eastern part of 
a cyclone come from the south, and are relatively warm; 
and as they advance into colder latitudes their vapor con- 
denses into cloud, rain, or snow ; while in the western 
part of cyclones the winds come from the north, are rela- 
tively cold, and as they enter warmer latitudes less con- 
densation takes place. Hence, as a cyclone approaches a 
place from the west, relatively warm, cloudy, rainy, or 
snowy weather prevails ; but as the center passes to east- 
ward over the place, a change to relatively cold, clear 
weather takes place. Anticyclones, on account of the re- 
versed direction of the whirl, have colder and clearer 
weather on their east than on their west sides; but as the 
air in an anticyclone is sinking, and hence becoming 
warmer, it frequently happens that little or no condensa- 
tion into cloud or rain occurs on either of its sides. 

The chart (Fig. 118) indicates the observed weather east of the 
Rocky Mountains one November morning. A large cyclone is 
central over Iowa (low). To the east of low the winds of the 
whirl blow from south or south-east; to the north of low, from 
east or north-east; to the west of low, from north or north-west; 
and to the south of low, from west or north-west. To the east of 



296 



PHYSICAL GEOGRAPHY. 




, Isobars, every fyo*-" 0/ an inch > Direction of wind & clear weather 

Isotherms, " 10 degrees — °— * " " •• •• cloudy „ 

"Low" = Center of Cyclone — •""* " " " " ra!n 

"High" = Anticyclones — ■-»■ " " " " snow 

<=£§> ■ V path of Cyclone 

Fig. 118. 

low, the southerly winds carry the warm air northward, so that the 
isotherm of 40 lies in the latitude of Cape Cod, Lake Erie, and 
southern Lake Michigan; to the west. of low, the northerly winds 
carry the cold air south, so that this same isotherm lies near the 
coast of Texas. To the east of low, the warm air is constantly 
getting colder as it moves northward, and its vapor condenses, first 
into clouds near the south and east edge of the cyclone, then' into 
rain as it reaches colder latitudes, and at last into snow as its tem- 
perature falls below the freezing point. Close to the west of low, 
the cold air from the north-west lowers the temperature, and the 
vapor still remaining in the winds from the north-east, is condensed 
into snow ; but some distance to the west of low, cold and clear 
weather prevails. This general distribution of the various kinds of 
weather over the Central States was predicted 24 hours previous, 
when the center of the cyclone was observed to be in Indian 
Territory, near the feather end of the long dotted arrow; and its 
present position in Iowa enabled the Signal Service to predict the 



WEATHER AND CLIMATE. 297 

distribution of weather which prevailed 24 hours later, when the 
cyclone center had advanced to the point end of this arrow. The 
decrease in pressure toward the extreme north-west corner of the 
chart indicates the approach of a cyclone from that direction. 
Experience with cyclones in that quarter teaches that they move 
south-east over the Rocky Mountains into Texas or Kansas, and 
thence north-east or east to the Great Lakes; and the kind of 
weather that their progress will cause in various localities may be 
predicted with considerable certainty at least 24 hours in advance. 

Climate. — If the weather at any locality be carefully 
observed for a long time, it will be found to repeat itself 
more or less exactly, each year. Some years may be un- 
usually hot or dry, and others may be exceptionally cold 
or wet ; but when many years are compared, the general 
similarity in the succession of weather one year with 
another, can not fail to be remarked. This average annual 
succession of weather peculiar to any locality, constitutes 
its climate. Climate, like weather, embraces all meteoro- 
logical phenomena ; but the factors most important to 
agriculture and hygiene are: (1) the mean annual temper- 
ature, (2) the mean annual rain-fall, and (3) the distribu- 
tion of sunshine, temperature, and rain-fall throughout the 
year. To the navigator another factor of equal impor- 
tance is the direction and force of the wind. 

The importance of the distribution of temperature and rain-fall 
through the year appears from a single example : San Francisco 
and Washington City have the same mean annual temperature 
(55 ); yet the Washington summers are 18 hotter, and the winters 
18 colder, — that is, the annual variation, or range, of temperature 
is 36 greater — than at San Francisco, where ice and snow in winter 
and oppressive heat in summer, are alike unknown. Sacramento, 
Cal., has only two thirds the rain-fall (22 inches) of Toledo, Ohio, 
(33 inches), and receives almost all of it in winter and spring, 
while at Toledo the rain-fall is nearly equal in each season, though 
slightly greater in summer and autumn. 

The latitude of a place is the most important factor in 
connection with its supply of heat. In consequence of 



298 



PHYSICAL GEOGRAPHY. 



100/; 

90/0 
80% 

60 % 

5°% 






















































































s 














1 





Fig. iid- 



the increasing obliquity of the sun's rays as the poles are 
approached, (page 50), the mean annual heating power of 
the sun's rays falling upon a given 
> °° °s i s i °s horizontal area, decreases from the 
equator to the poles in about the 
proportion indicated by the curved 
line in the diagram (Fig. 119), be- 
ing but YQ-ths as great at the poles 
as at the equator; and hence, in 
general, climates become colder as 
one journeys away from the equator. 

Effect of Latitude on Annual Range of Tempera- 
ture. — All places receive heat by day and lose heat by 
night. At the equator the days and nights are always of 
equal length ; hence, each night the temperature falls 
about as much as it rises during the day, and as the sun 
at noon is never very far north or south of the zenith, the 
heating power of its rays is nearly the same at all seasons. 
Therefore, the mean temperature of every day in the year 
is nearly the same ; consequently, in equatorial regions 
there is no thermal division of the year (into winter and 
summer), but the climate over the whole torrid zone is 
characterized by great uniformity of temperature, the 
greatest variation being, in general, that between day and 
night. This is seldom more than 18 , and at some places 
near the equator it is much less. 

At places not on the equator the lengths of the 
days and nights are constantly changing ; for six months 
the days are longer than the nights, and for six months 
the nights are the longer. When the days are the longer, 
a place receives more heat by day than it loses during the 
short night, and thus accumulating heat, its mean temper- 
ature rises for six months ; then, as the nights become the 
longer, it loses more heat than it receives, and its mean 



WEATHER AND CLIMATE. 



299 



daily temperature falls for six months. Now, the farther 
a place is from the equator, the greater is the difference 
between the length of its days and nights (page 51), and 
hence the greater is the variation of temperature during 
the year. In addition to this, the sun's rays, in middle 
and higher latitudes, are much 

more oblique, and their heat- I i s 1 ^ § & £ i g i 3 £ 
ing power is much less when 
the days are shortest than 
when longest; and this differ- 
ence increases as the distance 
from the equator increases. 
Therefore, the climate in tem- 
perate and polar latitudes is 
characterized by a great va- 
riation, or range, of temper- 
ature during the year which 
increases, in general, as the 
latitude increases. 







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Fig. 120. 



This is graphically indicated on the diagram (Fig. 120), which 
shows the average monthly mean temperatures at : 

Paramaribo, Guiana, latitude 5^°, Range 4°. 

Key West, Florida, " 24^°, " 14 . 

Portland, Maine, " 43^°, " 47°. 

Fort Conger, Arctic Regions, " 8i?{°, " 76 . 

The diagram also indicates that in summer, when the sun is 
nearly vertical over Key West, the temperature at that place is 
about 2 higher than in Guiana, nearer the equator. In winter, 
however, when the sun is over the southern tropic, the temperature 
at Key West is about io° lower than at Paramaribo ; hence, the 
mean annual temperature is lower at Key West than at places 
nearer the equator, though the summer temperature is higher. 

Effect of Land and Water Surfaces on Climate. — 

It has been explained (page 64) that a water surface tends 
to equalize temperatures, while a land surface undergoes 




TEMP ERAT U R E 



(300) 



WEATHER AND CLIMATE. 3OI 

extremes of heat and cold during the year; and since the 
air acquires its temperature largely from the surface on 
which it rests, these peculiarities are impressed upon the 
climates of the oceans and the land respectively. That is, 
the climates of inland localities invariably have a greater 
annual range of temperature than those of coast regions, 
or of the open ocean in the same latitude. 

Thus, the interior portion of the United States from El Paso, 
Texas, to North Dakota, has an annual range 25 to 55 greater than 
Jhe Pacific coast, and 6° to 30 greater than the Atlantic coast In 
corresponding latitudes. The summers of the interior are slightly 
warmer, but the winters much colder, than those on the coasts or 
oceans; hence, the interior has a lower mean annual temperature, 
except in equatorial latitudes, where, as explained (page 62), the 
land is warmer than the sea at all seasons. 

Continental and Oceanic Climates. — On account of 
the great influence of extensive land or water surfaces 
upon the variations of temperature, localities at which the 
annual temperature oscillates through a wide range are 
said to have a continental climate, while those where the 
range is small are said to have an oceanic climate. 

The fitness of these names is rendered apparent by the chart, 
upon which the pink tint deepens as the range (between the mean 
temperatures of the hottest and coldest months) increases. The 
regions where the range is less than 18 have no pink tint, and are 
seen to embrace almost the entire ocean, except the polar seas, 
where the range is greater on account of the high latitude. Almost 
all the land surface, on the contrary, is tinted pink, and has a range 
greater than 18 . The range increases inland, being about 72 in 
the interior of northern America, and 108 in the more extensive 
grand division of Euro-Asia. The only parts of the land where the 
range is less than 18 are certain coast regions, where the influence 
of the neighboring ocean is great, and the equatorial regions of 
the land where, it has been seen, the mean temperature of all the 
months is nearly the same; but even here the range between day 
and night sometimes greatly exceeds 18 in the interior of the con- 
tinents. 



302 PHYSICAL GEOGRAPHY. 

Climatic Differences of East and West Coasts. — 

In middle and higher latitudes (beyond 30 or 40 ), a 
marked difference of climate exists in corresponding lat- 
itudes between the east and west coasts of the continents. 
This is caused chiefly by the relation between the conti- 
nents and the direction of the winds. In these latitudes 
the general movement of the (antitrade) winds is from 
the west. In winter, however, on account of the dif- 
ference of temperature between the land and sea air, a 
great anticyclone tends to form over the cold interior of 
the continents, and a great cyclone over the warmer 
oceans. The course of the air in passing out of the anti- 
cyclone into the cyclone is such, in the northern hemi- 
sphere, as to make winds from the north-west prevalent on 
the east side of continents, and from the south-east or 
south on the west side at this season (see Wind Chart, 
page 87, January). Hence, the eastern winters are much 
colder than the western, since the east side is flooded with 
dry air from the intensely cold northern part of the 
interior, while the west side is covered with air from the 
relatively warm southern part. Furthermore, the winters 
on the east side are relatively dry, since the air is advancing 
into lower latitudes and hence becoming warmer. On the 
west side, however, the winters are relatively moist, since 
the air is advancing into colder latitudes and constantly 
increasing in relative humidity. In summer, on the con- 
trary, the relatively warmer air over the land tends to form 
a cyclone over the interior of the continents into which 
the surrounding air whirls, resulting in southerly winds 
on the east side, and northerly winds on the west side of 
the continents (see Wind Chart, page 87, July). Hence, 
the summers on the east side are relatively warm and 
moist, while those on the west side are relatively cool and 
dry. In general, the east side of continents in middle and 



WEATHER AND CLIMATE. 



303 



higher latitudes has a continental climate with abnormally 
low mean temperature, and the greatest rain-fall in sum- 
mer; while the west side has a moderately oceanic climate 
with abnormally high mean temperature, and the greatest 
rain-fall in winter. 





UNITED STATES 43 1 /2°LAT. EURO 


-ASIA 52° 
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West Coast Interior East Coast 



Fig. i2i. 



On the diagram (Fig. 121 ), this is well shown for middle latitudes 
in both North America and Euro-Asia. It is seen that while on the 
east coasts the range is not quite so great as in the interior, it is 
much greater than on the west coast, the summers being slightly 
warmer and the winters much colder ; hence, the mean temperature 
is lower. The range is greater in Euro-Asia than in North America 
because the grand division is larger and the seasonal winds stronger. 
The rain-fall curves show that on the east side of both grand divi- 
sions most of the precipitation occurs in summer, while on the west 
side most of it occurs in winter. In the United States, the Rocky- 
Mountains and the Colorado River roughly divide the east from the 
west side in the matter of winter and summer rain-fall. In latitude 
6o° N., the east coast of each grand division is about 20 colder than 
the west coast, and has a range about 35 greater. These differences 



304 PHYSICAL GEOGRAPHY. 

decrease southwardly and disappear within the tropics. This partly 
explains why the east coast of North America in middle and higher 
latitudes has a colder and more extreme climate than the opposite 
European shores in corresponding latitudes ; thus, New York City 
has a mean temperature 8° less, and a range 26 greater, than the 
opposite coast of Portugal. In the same way, the climate of our 
north-west Pacific coast is more moderate than that of the opposite 
Asiatic coast. For the same general reason (direction of prevailing 
winds), there is a similar though small climatic difference between 
the east and west coasts of peninsulas or islands, or of great inland 
lakes. Thus, Milwaukee, on the west shore of Lake Michigan, has 
a mean temperature 2° lower, and a range 4^° greater, than Grand 
Haven, Mich., in the same latitude and only 80 miles distant, but on 
the east shore of the lake. The precipitation is 16% greater at 
Grand Haven, and is greatest in autumn rather than in summer,. as 
at Milwaukee. 

Ocean currents come from warmer or colder regions, 
and hence bring water abnormally warm or cold for the 
latitude ; and this modifies the temperature of the over- 
lying air. If this air is brought by wind to the land, 
then one may say that currents influence the climate of 
adjacent coasts. The currents on the west side of all 
oceans move from the equator in tropical latitudes and are 
relatively warm, while on the east side of the tropical 
oceans currents move toward the equator, and are rela- 
tively cool. Hence, the east coasts of the continents in 
equatorial regions have a warmer climate than west 
coasts. The reverse is the case in the higher latitudes, 
where cold currents move toward the equator in the 
western part of the oceans, and warm currents move 
from the equator in the eastern part (page 137). 

Thus, the west coast of Africa, and the west coast of America 
from 40 N. to 40 S.,are abnormally cool, while the east coasts of 
America from South Carolina to Cape Horn, and the whole east 
coasts of Africa and Australia have abnormally warm climates, 
owing to adjacent ocean currents. It is only opposite the equatorial 
calms, in which the counter-current carries warm water eastward, 




(3°5) 



306 PHYSICAL GEOGRAPHY. 

that inter-tropical west coasts are not relatively cool. In higher 
latitudes (above 40 N. and S.) the warm currents wash the west 
coasts, and aid the west winds slightly in producing a moderate 
climate, while the cold currents adjacent to the opposite east coasts 
exercise an equal influence in depressing the climatic temperature. 

The amount of precipitation in coast regions also 
depends conjointly upon the direction of the winds and 
neighboring ocean currents. Warm currents — those flow- 
ing from lower latitudes- -tend to produce a large precipi- 
tation, since the air over them is nearly saturated and is 
abnormally warm. It is therefore cooled and part of its 
vapor condenses, when transferred in any direction from 
over the current. Cold currents tend to prevent precipi- 
tation for contrary reasons. 

The general distribution of rain-fall (see opposite chart and one 
on page 76), shows the connection between atmospheric precipita- 
tion, the temperature of the ocean currents, and the direction of 
the winds. It is seen that in the lower latitudes, to about the lat- 
itude of Cape Mendocino, Norfolk, Gibraltar, and Japan on the 
north, and Rio de la Plata, Cape of Good Hope, and south Aus- 
tralia on the south, the east sides of the continents enjoy east winds, 
are washed by abnormally warm currents, and receive the heaviest 
rain -fall. In higher latitudes, embracing the northern parts of 
North America and EurO-Asia, and the southern part of South 
America and Tasmania, we have west winds with warm currents 
off the west coasts, and the west sides of continents in these lat- 
itudes receive the heavy rain-fall. 

The only places in tropical latitudes where heavy rain-fall occurs 
on west coasts are close to the equator, where the equatorial counter- 
current brings warm water against these coasts, and in India and 
Farther India, where the seasonal winds are very strong, and the 
configuration of these mountainous west coasts is peculiarly adapted 
to cool the moist south-west monsoon of summer. 

In the torrid zone, where the great uniformity of 
temperature prevents a thermal division, the year is divided 
by the variation in the amount of rain-fall into a wet season 
and a dry season. Local peculiarities largely determine 











(307) 



308 PHYSICAL GEOGRAPHY. 

the time of occurrence of these seasons at different places, 
but in general the wet or rainy season occurs when the 
thermal equator crosses or lies in the vicinity of any lo- 
cality; and this follows more or less closely the passage 
of the sun through the zenith of the locality. Near the 
thermal equator the motion of the air is upward ; and as it 
cools in rising, its vapor condenses into clouds and rain. 

Since the sun is twice annually in the zenith of all places be- 
tween the tropics, there is a tendency toward two rainy and two 
dry seasons each year in the torrid zone ; but, excepting near the 
equator where nearly six months intervene between the passages 
of the sun through the zenith, the two rainy seasons merge into 
one, thus dividing the year into one moderately long wet season 
and one very long dry season. 

Influence of Elevation on Climate. — The climate of 
highlands every-where has certain general peculiarities 
which distinguish it from that of adjacent lowlands. 
Prominent among these are a lower mean annual tempera- 
ture, and a greater difference between the temperature of the 
air and that of the ground or surface objects. 

The air resting on highlands is less dense, is clearer, and con- 
tains less vapor than that resting on lowlands, and hence has fewer 
molecules to absorb the heat of the entering sunbeams by day or 
of the outward-passing earth radiations at night. Therefore, the 
highland air must in general be cooler than that resting on low- 
lands. This is well shown by the distribution of mean annual tem- 
perature of surface air in the United States, where the peculiar 
southward extension of the isotherm of 50 in the east and of 40 
in the west is caused by the highlands of the Appalachian and 
Rocky mountains respectively. But for the very reason that the 
rays lose less of their heat in passing through highland air, the 
arrival or departure of these rays produces a greater heating or 
cooling effect upon the ground and surface objects than in lowlands ; 
that is, the ground on the highlands, when exposed to the sun (by 
day or in summer), becomes hotter than the overlying air or than 
lowland ground. On the other hand, when not directly exposed to 
the sun (at night or in winter), the highland ground may become 




(309) 



3IO PHYSICAL GEOGRAPHY. 

colder than either the overlying air or the lowland ground (p. 21). 
Vegetation fails on high mountains (even near the equator), not 
because the sun does not supply sufficient heat, but because the 
evaporation is too great, and the rare, dry air can not retain the 
heat near the earth's surface, and thus allow it to accumulate from 
day to day. In the lowlands of polar regions, on the contrary, 
vegetation does not thrive, partly because the sun's rays fall so 
obliquely that, though the dense lower air permits the heat to accu- 
mulate during several months of constant day, the aggregate is 
only sufficient to support a stunted vegetable life. 

The exposure, or direction of slope, in hilly country- 
has a great influence on the amount of heat imparted to 
the ground, and hence upon the climate. In the northern 
hemisphere the southern slopes receive the rays more per- 
pendicularly, and for a longer time each day, and are 
hence warmer than the other slopes. In the southern 
hemisphere the northern slopes are the warmest. The 
higher temperature of the ground affects the overlying air, 
and makes the climate more moderate. Therefore, other 
things being equal, the lower limit of perpetual snow, 
and the higher limit of vegetation, lie at a greater height 
on the south than on the north slopes of mountains in 
our hemisphere. 

The rate at which the air becomes cooler as the 
observer ascends, varies at different places, and at different 
times at the same place. The general average is about 
i° Fahr. for each 300 to 350 feet of elevation on a slope 
whose acclivity is quite steep, as on a mountain side, but 
it is sometimes as rapid as i° for every 200 feet, and 
sometimes as slow as i° for every 500 feet. The rate is 
generally most rapid in summer, and on the warm side of 
a mountain. The rate is much slower on gentle acclivities 
than upon steep slopes. On the ordinary slopes of non- 
mountainous regions, as the great Mississippi Valley, the 
average rate is about i° for each 450 feet of ascent. 



WEATHER AND CLIMATE. 3II 

Peculiarities of vertical distribution of temperature 
in hilly regions. During calm, clear nights, especially in 
winter, in middle and higher latitudes, it is observed that 
up to a certain height the air in valleys is colder than that 
on the slopes of surrounding eminences. Over open 
plains it has also been observed that the temperature dur- 
ing calm, clear nights increases with elevation. This in- 
crease of temperature extends at least to a height of 150 
feet, and is most rapid in the lowest layers of air, where 
it may attain a rate of i° in 5 feet, or even more. Thus, 
on frosty nights the tree-tops frequently remain unharmed, 
while the lower foliage and herbage are frozen. 

The earth cools quickly on clear nights by radiating its heat 
hrough the overlying air. The air cools much less except where 
dusty or damp enough to have effective radiating power. Thus, the 
lower air is chilled by contact with the colder earth. On valley 
slopes the cooled and hence heavy surface air creeps down to the 
lowest ground, where it accumulates, lifting up the relatively warm 
air that it finds there. Accordingly, there is a climatic tendency 
toward warmer nights on slopes and hill-tops than in adjacent val- 
leys. In latitudes where frosts, though infrequent, sometimes occur, 
this peculiarity is of great importance to the agriculturist, since the 
frosts, though occurring in the valleys, may never occur on the 
higher grounds. A region called the Thermal Belt, in the Appa- 
lachian Range, is thus specially favored. 

Mountains tend to produce condensation of atmos- 
pheric vapor in all parts of the world, since the lower and 
moister air-currents are compelled to ascend in crossing a 
mountain range, and are thus cooled by expansion. 
Hence, mountain slopes to a certain height usually have a 
moister climate, that is, they have more clouds and rain, 
than the surrounding lowlands. Thus, in the plateau re- 
gion of the West, many of the mountain ranges and higher 
mesas have a sufficient rain-fall to support quite a heavy 
growth of forest, while on the lower general surface of 



312 PHYSICAL GEOGRAPHY. 

the country, the rain-fall is so slight that prairie grass, 
sage-brush, and cactus are the only forms of vegetation, 
except along the streams that carry off the surplus rain- 
fall of the mountains. Even in the center of the intensely 
dry desert of Sahara, the higher mountain regions of 
Asben and Tibesti have a regular summer rain-fall. 

Mountain ranges have a moist side and a dry side 
when they trend more or less directly across the direction 
of the prevailing winds. In the torrid zone, where 
easterly winds prevail, the east slope is usually the moist 
side, — as, for instance, the American Cordilleras from 
Mexico to northern Chile. In higher latitudes the west 
side of mountain ranges usually receives the greatest rain- 
fall, — as, for examples, the Cascade Range in Oregon and 
Washington and the Andes of southern Chile. Mountains 
whose trend is nearly parallel with the course of the wind, 
as the Appalachians and the Alps, have no well marked 
wet and dry sides. 

In crossing a mountain range, the air loses by condensation on 
the windward slopes all the vapor it contains in excess of the 
amount which saturates it at the lowest temperature it attains when 
near the crest. In gradually sinking on the further, or lee, side of the 
mountain, the air is- mechanically warmed, and hence its relative 
humidity decreases. This not only produces an excessively dry 
climate, but operates, also, to raise the mean, and increase the range 
of temperature on the lee side of the mountain, for the dry air and 
cloudless sky favor intense heating of the earth's surface by day, 
and rapid cooling by radiation at night, while on the windward side 
the rising air favors the formation of clouds and mists, which pre- 
vent intense heating of the earth by day, or extreme cooling by 
night. 



PART VI.— LIFE. 
CHAPTER XXII. 

THE VARIOUS FORMS OF LIFE. 

My substance was not hid from thee, "when I was made in secret, and ciiriously 
'wrought in the lowest parts of the earth. Thine eyes did see my substance, yet be- 
ing imperfect ; and in thy book all my members were written, which in continuance 
were fashioned, when as yet there was none of them. — Psalm cxxxix : 15, 16. 

Life is a mysterious and temporary manifestation in a 
peculiar kind of matter. This kind of matter is called 
protoplasm. The chemical composition of this substance 
is very imperfectly understood, but it is known to consist 
chiefly of carbon, oxygen, hydrogen, nitrogen, and sul- 
phur, in various combinations, which differ somewhat in 
different kinds of protoplasm. But in all kinds, certain 
highly complex compounds of these substances, called 
proteids, are practically identical, and only matter in which 
these proteids are present is known to manifest the prop- 
erties of life. 

All matter in the living state is closely associated with lifeless 
matter in the same body or structure ; thus, the fat, parts of the 
hair, nails, and blood, most of the horns or shells of living animals, 
and the bark, solid wood, and sap of living plants, are composed 
of matter in a perfectly lifeless condition. Science has never dis- 
covered what causes this wonderful difference of condition in mat- 
ter, but so far as we know, the living state is never assumed except 
under the influence of existing living matter, which seems to infect 
lifeless protoplasm, and in some way causes it to assume the living 
state. 

(313) 



314 PHYSICAL GEOGRAPHY. 

Living matter is distinguished from lifeless matter 

(1) by its power of repairing its waste, and of growth, and 

(2) by its power of reproduction. While a mass of mat- 
ter is in the living state, portions of it are constantly 
dying and being thrown off, but the living portion contin- 
ually repairs the loss by a process called intussusception. 
This consists in manufacturing appropriate kinds of new 
particles, and fitting them into the interstices between 
those present, throughout the whole mass. If this proc- 
ess exceeds the loss, the living mass increases in size, or 
grows. In this respect, living matter differs widely from 
lifeless matter, which grows, if at all, only by the addi- 
tion of particles to its surfaces. Living matter not only 
repairs its waste, and grows, but under certain circum- 
stances detaches from itself masses of living matter which 
are endowed with all the properties of growth and repro- 
duction possessed by the parent mass. 

Organisms. — Living bodies of all but the lowest forms 
are composed of unlike parts, each capable of performing 
different functions essential to the life of the whole body. 
These unlike parts, such as the stomach, heart, limbs, 
etc., in animals, and roots, stem, leaves, etc., in plants, 
are called organs, and the whole body is called an organ- 
ism because it possesses them ; while lifeless protoplasm 
is frequently called organic matter, because, so far as 
known, it has invariably been produced in living bodies. 
Cells. — All organisms exist at first as a minute mass 
of protoplasm called the germ-cell (Fig. 122), 
forming part of the body of the parent, from 
which it becomes detached when the new 
organism has reached the proper stage of its 
Fig. 122. development. The protoplasm of the germ- 
cell (p) is a transparent, jelly-like mass resembling white 
of egg, and part of it is usually gathered into a darker, 




THE VARIOUS FORMS OF LIFE. 



315 



rounded nucleus {it), while the whole may or may not be en- 
veloped in a membrane or sack of soft, lifeless material 
forming the cell-wall (m). When sufficiently magnified, the 
living protoplasm is seen to be always in motion, regular 
currents traversing its mass in more or less definite direc- 
tions. 

The simplest forms of life are organisms similar to 
the germ-cell, leading an independent existence, and re- 
producing similar forms by simply dividing into two or 

Fig. 123. — Various Stages in the life of 




AMOEBA 



more similar masses of protoplasm. Such simple yet 
complete organisms are the Protococcus and common Yeast 
plants, and the Amoeba animalcule. 

In all higher forms of life, the germ-cell develops 
by subdivision, or segmentation, through two, four, eight, 
sixteen, etc., into a great number of nucleated cells of 
protoplasm within the original cell- wall, which finally dis- 
appears. To this point the new cells closely resemble 
each other, being all nearly spherical, or varying from 
that form only by their pressure against one another. But 

P. G.— 18. 



i6 



PHYSICAL GEOGRAPHY. 




Fig. 124. — Segmentation of a Cell. 



the further development of the organism is still more 
wonderful. New cells continue to be formed by seg- 
mentation, but the cells in different parts of the mass 

begin to surround them- 



selves with cell-walls of life- 
less matter, and to adapt 
themselves for the various 
kinds of work they have to 
do, by gradual differentiation; 
that is, assuming different 
shapes and structures. In 
many parts of the organism 
the protoplasm may nearly or entirely disappear in the 
production of cell-walls, thus producing a lifeless but solid 
or cellular portion of the organism, as the woody part of 
plants, and the bones and outer skin of animals. By con- 
tinued segmentation and differentiation of the originally 
similar cells, the very dissimilar organs of the organism 
are finally formed, and each organ, when fully developed, 
is thus entirely composed of variously modified cells of 
living protoplasm, separated by more or less cellular walls 
of lifeless matter of various thickness, shape, and sub- 
stance. 

Respiration. — The development and growth of every 
organism as a whole is thus the result of the death and 
destruction of portions of its protoplasm. The general 
process by which this destruction is accomplished is the 
same in all organisms ; they all exhibit the phenomenon 
of respiration, ox breathing. Land organisms inhale atmos- 
pheric oxygen directly, while aquatic organisms inhale that 
which is dissolved in the water. The strong chemical 
affinity of oxygen for all other elements enables it to de- 
compose the complex protoplasmic substances of the 
organism and form simpler and more stable compounds, 



THE VARIOUS FORMS OF LIFE. 317 

organic energy and heat being liberated by the change. 
One of the stable compounds is carbonic acid, which is 
largely expired, or breathed out, by all organisms. Thus, 
respiration is directly a destructive process, since it results 
in the killing and removing of portions of the organism. 
Animal and Vegetable Kingdoms. — The material 
with which the loss, occasioned by respiration, is continu- 
ally repaired, is manufactured from the food of the organ- 
ism, the process being called nutrition; and since it 
consists of the conversion of lifeless food into living pro- 
toplasm, it is a constructive process, and therefore directly 
opposed to respiration. It is in connection with nutrition 
that the essential difference between plants and animals 
occurs. Since all living protoplasm contains the proteid 
combinations of carbon, oxygen, hydrogen, nitrogen, etc., 
the food of all organisms must contain these substances; 
but plants alone are able to manufacture the complex pro- 
teids out of simpler and more stable combinations of 
these elements, while animals require food in which the 
proteids exist already manufactured. Hence, the animal 
kingdom depends absolutely upon the vegetable kingdom 
for its food. 

All green plants, which form by far the larger portion of the 
vegetable kingdom, can manufacture their food only in the sunlight 
(direct or diffused), and these plants obtain their food chiefly from 
two sources : carbon they obtain mostly from the carbonic acid in 
the air, through minute mouths (stomata) in the under side of the 
leaf; hydrogen and oxygen are derived chiefly from the water ab- 
sorbed by the roots, though plants, like animals, also obtain oxygen 
by respiration from the atmosphere ; nitrogen, sulphur, and other 
elements are derived chiefly as various salts dissolved in the water. 
By the aid of the kinetic energy in, the sunlight, these green plants 
are enabled to decompose the water and carbonic acid, returning to 
the atmosphere part of the oxygen thus disengaged, but uniting the 
hydrogen and carbon with the rest of the oxygen to form a carbo- 
hydrate — starch. Sooner or later this is changed into a kind of 



31 o PHYSICAL GEOGRAPHY. 

sugar (glucose), and, dissolved in the sap, is transferred to the point 
where new protoplasm is needed. Here, in some unknown way, it 
unites with the nitrogen and sulphur to form a proteid, and the 
newly made protoplasm becomes endowed with the properties of 
life. A few plants, as the fungi, bacteria, and common yeast-plant, 
do not require sunlight, but can live in darkness. These plants, 
like animals, require organic food ; but, unlike animals, can manu- 
facture proteids, if only a carbohydrate is present in their food. In 
this respect these organisms occupy an intermediate position be- 
tween the animal and vegetable kingdoms. In almost all animals 
the region where nutrition occurs is completely and more or less 
directly inclosed by layers of cell-walls or membranes, but the pro- 
teids in food are indiffusible ; that is, unable to pass through a mem- 
brane. Hence, the food has to be made diffusible before it can 
enter the system. It is to effect this preparatory change in the 
food, called digestion, that animals require a stomach, which is es- 
sentially a more or less complicated pouch formed by the infolding 
of the outer surface of the body. Thus, it is only after the digested 
food has passed through the walls of the stomach that it really 
enters the body. Plants, which manufacture the proteids within 
themselves, and some of the lowest animals, which, being minute 
naked masses of protoplasm, can receive their food by simply flow- 
ing over and enveloping it, of course require no stomach. 

Two great laws of the organic world have been es- 
tablished from prolonged observation of living things: 1st, 
The Law of Heredity ; organisms reproduce others, which at 
maturity closely resemble their parents. Though the resem- 
blance is close, the likeness is never exact, and this leads, 
2d, to the Laxv of Adaptation; all organisms possess, in 
greater or less degree, the power to adapt themselves to 
gradual changes in their surroundings, or environment. 

It is a well known fact that family resemblances may generally 
be traced from one generation to another, but no two human beings 
are so exactly alike in all particulars that intimate friends can not 
distinguish certain differences. The same is true of all animals and 
plants : there is a close resemblance running through the various 
families, but no two organisms are exactly alike, though people gen- 
erally are not sufficiently well acquainted with them to recognize at 
once individual peculiarities. ' The power of adaptation is illustrated 



THE VARIOUS FORMS OF LIFE. 319 

not only by the alteration in the appearance of plants and in the 
quality of the fur of many animals as the seasons change, but by 
the alteration in the skin and muscles of men and women which 
follows certain changes in their mode of living, as from an indoor, 
inactive life, to one of hard manual labor and exposure to the sun 
and elements. 

The environment of an organism embraces every- 
thing outside of itself that affects in any way the condi- 
tions of its existence. It embraces (1) all the factors that 
influence the food supply of the organism ; (2) all the 
factors of climate ; (3) all the factors that determine the 
presence or absence of other plants or animals that inter- 
fere with or promote the well-being of the organism ; and 
(4) every thing that modifies any one of these factors. It 
is inconceivable that all these factors can ever be exactly 
alike at two different localities or at two different times. 
Hence, every organism has a different environment which 
is constantly changing to a greater or less extent, and the 
constant adaptation of an organism to its special environ- 
ment probably accounts for its individual peculiarities. 

Classification. — The grouping of organisms according 
to the degree of similarity in structure or function of their 
corresponding parts, constitutes classification. The first 
and broadest grouping of living things is into the vegeta- 
ble and animal kingdoms. Each kingdom is then divided 
into several smaller groups, and these into others, which 
in turn are subdivided again and again. Each of the two 
largest divisions embraces organisms which are widely dis- 
similar in almost every respect excepting mode of nutrition, 
while each of the successively smaller groups is character- 
ized by a greater and greater number of similarities be- 
tween the organisms of which it is composed, until, in the 
smallest groups, of which there may be a million or more, 
all the individual organisms of each group, while not 
exactly alike, resemble each other so closely in structure 



3 2 ° 



PHYSICAL GEOGRAPHY. 



and function that they are said to constitute a single kind, 
or species, of plants or animals. Thus, there are a number 
of varieties of apples, and yet they are all sufficiently 
similar to be classed as a single species of the vegetable 
kingdom ; and in the same way all chickens, though no 
two are exactly alike, are essentially similar, and are 
classed as a single species of the animal kingdom. 

The characteristic similarities which determine some of the larger 
groupings and subgroupings of the organic world are given below, 
the groups embracing the simplest or lowest forms of vegetable and 
animal life respectively being placed first, and those containing the 
most complex or highly organized forms being placed last. 

Vegetable Kingdom, 

all organisms able to mamtfacture the complex proteids. 

a. Cryptogamia {hidden seeds). All flowerless 
plants. Subdivided into : 

i. Protophytes {first plants). Simplest and lowest 
plants. Generally microscopic ; either single cells or 
an association of cells without mutual dependence, as 
the diatoms, moulds, bacteria, yeast, etc. (Fig. 123). 

2. Thallogens {shoot growers). Many cells, but 
without differentiation into stem and leaf; growing 
horizontally in spreading shoots or fronds, as the 
alga, or sea-weeds ; fungi, or toad-stools (Fig. 125) ; 
and the lichens. 

3. Bryogens {moss growers). Cells differentiated 
into root, stem, and leaf, but no woody material; 
showing tendency to grow upward rather than hori- 
zontally, as the liverworts and mosses (Fig. 126). 

4. Acrogens {highest growers). Cells differentiated 
more completely, the stem and leaves containing 
vascular, woody fibers; showing strong tendency to 
grow upward, as the ferns (Figs. 127, 129). 

b. Phenogamia {visible seeds). All flowering plants. 
Subdivided into : 

1. Gymnosperms (««M 5«^). Flowering plants 
127. which do not inclose their seeds in seed-vessels. The 




125. 



126. 




THE VARIOUS FORMS OF LIFE. 



321 



group is subdivided into the (a) cycads (palm ferns, Fig. 128), (b) 
conifers (pines, firs, spruces, larches, cypresses, cedars, etc.), and 
(c) gnetums. 



2. Angiosperms {seed-vessels). 
their seeds in seed-vessels. The 
group is subdivided into (a) 
monocotyledons (single lobed), 
which first develop a single seed 
leaf, or lobe, and are character- 
ized by leaves having parallel 
veins ; by three-petaled flowers ; 
and by the absence of a distinct 
pith and lines of annual growth 



Flowering plants which inclose 




Fig. 129. 



in the stem, as the rttshes, grasses, (cereals, corn, cane, etc.), lilies, 
bananas, and true palms (Fig. 130) ; (b) dicotyledons (double lobed), 
which first develop a pair or more of seed leaves, or lobes, and are 
characterized by leaves having netted veins; and by the division of" 
the stem into a central pith, an outside bark, 
and a series of concentric layers of wood be- 
tween them, an additional layer of wood being 
added beneath the bark by each season's 
growth. This group includes most garden 
vegetables, fruit- trees, a?id hard -wood forest 
trees. It is subdivided into : (a) monochlamyds 
(single cloaks), or plants whose flowers consist 
of but a single whorl of leaves (the calyx), em- 
bracing the catkin-bearing plants, as willows, 
poplars, beeches, oaks, elms, laurels, hemp, 
hops, etc.; and (b) dichlamyds (double cloaks) 
or plants whose flowers consist of a double 
whorl of leaves (the calyx and corolla). This subdivision embraces 
most garden vegetables, cultivated flowers, fruit-trees, the locust, ash, 
elder, etc. 

Animal Kingdom, 

all organisms reqtiiring proteid food. 

i. Protozoa {first life). The simplest animals; mostly micro- 
scopic; consisting of a single cell, with or without nucleus; no 
stomach or organs, as the amceba (Fig. 123) and other animalcules, 
the radiolarians and foramenifers found in the oceanic oozes, infu- 
sorians, etc. 




*;-&m 



^-^ 



322 



PHYSICAL GEOGRAPHY. 




Fig. 131. 



2. Porifera {pore bearers). Animals having many cells but no 
special organs. These animals are traversed by many pores, or 
cavities, which serve the purpose of a simple 
stomach. Though possessing no fixed symmetry 
of form, most of these animals secrete a stony 
or horny substance from their food, which serves 
the purpose of an irregular frame-work or skel- 
eton. Such animals are the sponges (Fig. 131). 

3. Ccelenterata {hollow stomached). Animals 
possessing a single, distinct stomach-cavity, with 
a body-cavity extending off from it. In this cav- 
ity or elsewhere several distinct organs appear; 
a more or less distinct symmetry of form, similar 
parts of the body being usually arranged around 
a center, like the spokes of a wheel around the 
hub. Such animals are the hydras ; medusa, ox 
jelly-fishes (Fig. 132); and the corals (Fig. 68). 

4. Echinodermata [spiny or rough skinned), having true stomach, 
separate from another body - cavity, containing 
organs answering to a heart and nervous system; 
radial symmetry like the preceding, but each ray 
usually consists of two similar halves, placed side 
by side (bilateral symmetry). Such are the crin- 
oids, star-fishes, sea-urchins (Fig. 133), and sea- 
cucumbers. 

5. Vermes {worms). Lowest animals possess- 
ing clearly bilateral symmetry ; stomach divided 
into various special parts ; body composed of a 
series of rings or segments ; distinct head containing nervous cen- 
ters (ganglia), such as the common angle-worm. 

6. Mollusca {soft). Soft, 
unsegmented bodies, bilat- 
erally symmetrical, e n - 
veloped by a leathery 
mantle, which usually de- 
velops a hard shell-cov- 
ering, or external skel- 
eton ; a symmetrical nerv- 
ous system, consisting of 
several connected nerve 




Fig. 132 






r\ 



Fig. 133. 



THE VARIOUS FORMS OF LIFE. 



32.3 




*/m w^rs 




bunches, or ganglia. Such are the clams, oysters, muscles, and 
snails (Fig. 134), conchs, cuttle-fishes, the nautilus, etc. 

7. Arthropoda {jointed feet). Bilaterally symmetrical bodies 
composed of a series of rings or segments, each of which bears a 
pair of jointed appendages, or limbs; a well 
developed and symmetrical nervous system 
of many connected ganglia. The lower ar- 
thropoda are the crustaceans — barnacles, lob- 
sters, (Fig. 135), crabs, etc. The higher are 
the insecta, or insects, as spiders, myriapods, 
grasshoppers, beetles, flies, moths, butterflies, 
bees, wasps, ants, etc. 

(8) Vertebrata {flexible). Animals possess- 
ing a flexible backbone and an internal skel- 
eton (see Fig. 146) ; a true brain in the head 
(see Fig. 148); a spinal nerve cord; and a i-*g. 134. 

more or less highly specialized nervous system. 

The subdivisions of this, the highest 
of the primary groups of animals, 
are (a) fishes, (b) amphibians, (frogs, 
toads, etc.), (c) reptiles, (snakes, 
lizards, etc.), (d) birds, and (<?) 
mammals (the breast), or animals 
which feed their young from the 
breast. The highest of these groups, 
-the mammals, is divided into three 
subgroups, the monotremes or lowest, including but a few rare 
mammals found about Australia (Fig. 140), having a body and skel- 
eton much like a mole or hedgehog; a fiat bill and web feet like 
a duck or alligator ; and which hatch their young from an egg like 
a bird or reptile. The next higher subgroup, the marsupials (Figs. 
140, 145), includes a greater number of mammals, but, with the ex- 
ception of the opossum of America, they are all found in Australia 
and the neighboring islands. These animals bring forth their young 
alive — that is, after the egg-envelope is broken — but the young are 
brought forth in such an imperfect condition that for some time after 
birth they are carried, attached to the breast of the mother, in a 
pouch, or fold, of skin with which she is provided. The highest 
of the three subgroups include all the rest of the mammals, from the 
armadillos, ant-eaters, and sloths, which have the lowest and sim- 
plest, to man* who has .the highest and most complex, organization. 




Fig. 135. 



324 PHYSICAL GEOGRAPHY. 

By comparing the characteristics of these great 
primary groups, one is immediately struck by the fact that 
in both the vegetable and animal kingdoms the organisms 
show a progressive but gradual complication of structure, 
and a corresponding specialization of function in their 
several parts or organs ; from low, independent organisms 
without definite structure, consisting simply of minute 
masses of protoplasmic jelly, all parts of which possess 
equal ability to perform all the duties essential to con- 
tinued life, they increase to forms of highly complex struct- 
ure, possessing many distinct organs, each adapted to a 
special function. 




Amoeba Sponge Crab Trout Chicken Cat 

{Protozoa) {Pori/era) (ArtkroJ>oda) ' Vertebrata. ' 

Fig. 136. Mature amceba,and germ-cells of successively higher groups. 

A progressive complication of structure similar to 
that exhibited by the great primary groups, occurs also 
during the very early, or embryonic, development of every 
individual in the higher groups of organisms. As already 
stated, every organism, even the highest and most com- 
plicated, is at first a simple germ-cell which often can not 
be distinguished from the mature single-celled organisms 
of the lowest groups (Fig. 136). The embryo, in acquir- 
ing the complicated structure peculiar to its own group, 
passes in succession through stages in which it closely re- 
sembles more nearly mature organisms of lower groups. 

In Fig. 137 is shown a section of an embryo animal in each of 
the primary groups, at the gastrula stage. This stage occurs just 
before maturity in the sponge, but at successively younger periods 
in the higher groups. Fig. 138 shows successively younger em- 
bryos from successively higher groups of the vertebrata. 



THE VARIOUS FORMS OF LIFE. 



325 




Sponge Coral Star-fish 

(Porifera) (Calenter.) (Echinoderm.) 



Worm Snail 

( Vermes) (Mollusca) 

S. indicates stomach cavity. 

Fig. 137- 



Barnacle Lancelet 

(Arthropoda) ( Yertebrata) 




Trout 

(Fish) 



Frog 

(Amphibian 



Turtle Chicken 

(Reptile) (Bird) 

Fig. 138. 



The fossil remains of ancient forms of life indicate 
that in general the more complex and specialized organ- 
isms appeared on the earth at successively later dates in 
its history. The oldest rocks in which metamorphism has 
not so nearly obliterated the fossils as to render their 
original form quite unrecognizable, are those of the Cam- 
brian period. In these rocks, fossil thallogens (sea-weeds) 
are found, but no forms of any higher vegetable groups. 
Fossil representatives of most of the lower animals are 
also found, but none of any higher group than artJiropoda. 
It is not until late in the succeeding Silurian period that 
fossils of a higher group of plants and animals {acrogens 
and vertebrates — fishes) first appear. But fishes are the low- 
est group of the vertebrata. At successively later periods 
fossils of higher groups are found in the order of their 
complexity (gymnosperm plants and amphibians and rep- 
tiles), and at still later periods the still higher groups of 



326 



PHYSICAL GEOGRAPHY. 



birds, mammals, and angiosperm plants, while the earliest 

remains of the highest organism, man, do not occur until 

a very much later period. 

In the interval between the appearance of successive groups, 

fossil forms, now extinct, are occasionally found, that show a 

structure intermediate be- 
tween the two groups. 
Thus, the fishes are linked 
to the amphibians, and the 
reptiles to the birds, by ex- 
tinct fossil forms curiously 
blending the structures of 
both groups ; while the 
lowest Australian mam- 
mal, with its peculiar web- 
footed structure and curi- 
ous functions of laying 
eggs and yet suckling its 
young when hatched, is 
sometimes called a "liv- 
ing fossil," being really a 
link between the reptiles 
and the mammals. 

The Development 
Theory. — Three great 
Fig. 139. facts are thus presented 

to us: (1) living organisms increase more or less gradually 
in complexity of structure, from the lowest to the highest; 
(2) there is a gradual complication of structure of each or- 
ganism during its development, from a simple germ-cell 
resembling the lowest forms, through stages resembling 
successively higher forms ; and (3) fossils of successively 
more complicated forms of life appear at successively 
later periods of time. A consideration of these facts 
has led to the belief that life first appeared upon the 
earth at an inconceivably remote period in the past, as a 
single kind or a very few kinds of organisms of the 







Relative age of various groups 


relative 

length 

of various 

Geological 

Periods 


Plants 


Animals 


1 1 
nil '| 


"■! fc < a a s s 


H 


PlioceDe 






" 














Miocene 






i ' J 














Eocene 




1 1- l"=o 














s 


Cretaceous 




« o'rT °. 
1 ac> I 








































i» i_ 1 I 












Cm 

>> 
£ 


















Carboniferous 


I 


u 


1 








. 


Devonian 


i 










Silurian - 


</ 


) 










Cambrian 










Archrrnn 











THE VARIOUS FORMS OF LIFE. 327 

simplest structure ; and that as age succeeded age, different 
descendants of these organisms encountered different 
changes in their environment, and thus were differently 
altered in structure and function, each to conform to its 
peculiar surroundings, each generation undergoing, through 
its power of adaptation, a very slight change of form, and 
transmitting it to the next generation, through the power 
of heredity. As countless generations followed one another, 
the imperceptible changes in each gradually produced or- 
ganisms differing widely in both structure and habit, or 
function, from their remote ancestors ; and as the surround- 
ings of different organic groups differed greatly, so did the 
resulting organisms differ from one another as widely as 
from their common ancestors. Thus, it is conceived, has 
arisen not only the infinite variety in the organic forms 
inhabiting the world, but the remarkable adaptation of 
each form to its special surroundings, or environment. 

Great, though exceedingly gradual, changes of environment 
would naturally ensue (1) from the gradual transference of organ- 
isms into new localities, by winds, currents, other organisms, or their 
own powers of movement ; (2) from gradual changes of climatic 
or other conditions resulting directly or indirectly from the cooling 
of the planet, the slow movements of the earth's crust, or the 
equally slow effects of erosion ; and (3) from the sharp competition 
for food and other necessities (air, moisture, light, etc.), resulting from 
the continued multiplication of organisms in the same locality. It is 
probable that such competition has tended more than any other one 
cause to produce the endless variety and the progressive complexity 
of organic forms; for, where competition is sharpest, only those 
organisms that are most perfectly fitted, or adapted, to their sur- 
roundings get a sufficiency of the requisites of life, while the less 
perfectly adapted organisms have to migrate or perish. The favor- 
able peculiarities of the survivors are inherited by their descend- 
ants, and in a few generations become common. Then individuals 
possessing some new peculiarity become the favored and successful 
form. 



CHAPTER XXIII. 

DISTRIBUTION OF LIFE. 

/ will plant in the wilderness the cedar, the acacia tree, and the myrtle, and 
the oil tree; I will set in the desert the fir tree, the pine, and the box tree together : 
that they may see, and know, and consider, and understand together, that the hand 
of the Lord hath done this. — Isaiah xli : 19, 20. 

In general, the number of all forms of life decreases 
with the temperature and moisture of the climate. Thus, 
in the equatorial regions, where heat and moisture are 
great and continuous throughout the year, the luxuriance 
of both animal and vegetable life is astonishing. Dense, 
continuous, and evergreen forests are the striking feature 
of the vegetation. The trees not only stand very close 
together, but their trunks and branches are entwined with 
huge climbing vines, which, stretching from tree to tree, 
and interlacing with tall, tree-like ferns, grasses, and other 
plants, present an almost solid mass of vegetation often 
quite impassable by man. It forms a congenial home, 
however, for myriads of animal forms — mammals in great 
variety, birds of gorgeous plumage, reptiles of many 
kinds, toads and frogs, snails and other land shells, to- 
gether with hosts of butterflies, moths, beetles, and other 
insects. 

In passing from such regions to polar latitudes, one 
may observe that the forests gradually become more open, 
and the undergrowth less dense, while the animal forms 
become less numerous and less varied. As latitudes are 
reached in which the difference between winter and sum- 
(328) 



DISTRIBUTION OF LIFE. 329 

mer temperatures is more marked, the broad-leafed plants 
that were evergreen nearer the equator become deciduous, 
that is, they lose their leaves on the approach of winter; 
while almost the only remaining evergreens have needle 
leaves, like the pines, or plate -like foliage, like the 
cedars. This change in vegetation is accompanied by a 
corresponding change in the form and habits of animals. 
With the seasonal stoppage of plant growth, many of the 
animals are forced to migrate into warmer latitudes in 
search of food ; others are adapted to pass the winter in a 
nearly unconscious state in some secure retreat, under- 
ground, in hollow trees, or elsewhere ; while many take 
on a thicker covering of hair, fur, or feathers at the ap- 
proach of winter. 

In still higher latitudes, even such hardy trees as 
pines, firs, larches, and spruces can not withstand the cold 
winters ; all trees and even tall shrubs disappear, and in 
the polar lands only such low plants can mature their 
seeds during the short growing season as can be safely 
covered by the snow during the long winter, such as 
mosses, lichens, and strangely dwarfed and stunted shrubs. 
The animals of these cold latitudes are comparatively few, 
and, like the plants, are specially adapted to withstand the 
severe climate. 

The low northern margins of both America and Euro-Asia lie 
partly in this cold region, and the ground is frozen to a great depth. 
Only a thin layer is thawed during the short summer. The water 
from the melting snow, unable to penetrate the frozen ground be- 
neath, converts the flat region into a great morass, or tundra, in 
whose shallow soil the mosses, lichens, and other low forms of vege- 
tation peculiar to the region find sufficient foothold. 

Variation with Altitude. —A very similar series of 
changes in the organic world may be observed in ascend- 
ing a very lofty mountain range in the torrid zone. As 
the temperature falls with the ascent, the rank vegetation 



330 PHYSICAL GEOGRAPHY. 

and exuberant life of the lowlands are replaced by organic 
forms, which remind one of those to be found in the low- 
lands of colder latitudes. Finally, an altitude is reached 
where tall tree-growth ceases, and is succeeded by a 
region in which the vegetation consists only of mosses, 
lichens, and stunted shrubs. As the vicinity of the snow 
line is reached even these fail, and all forms of life are left 
below. 

Effects of Moisture — Forests. — But the organic 
forms do not depend upon temperature alone. Forests, 
with their .peculiar forms of animals, occur only where the 
rain-fall is abundant throughout the growing season. In 
regions where the rain-fall is but moderate, or very unequal 
at different seasons, the forests are replaced by the low veg- 
etation peculiar to open meadows, or pastures. 

Open Meadows. — Plains having this form of vegeta- 
tion are called prairies, steppes, llanos, pampas, and cam- 
pos in different regions. With the change of vegetable 
forms from forests to meadows,, a corresponding change 
takes place in the animal forms. Those common in the 
forest are rarely seen, but are replaced by other forms 
better suited to the open country and the altered condi- 
tions of climate. 

The meadow vegetation may be evergreen in low latitudes, where 
the annual temperature is uniform, if the rain-fall is equally distrib- 
uted through the year ; but where precipitation is very intermittent, 
the vegetation is burned crisp and brown, and killed to the roots by 
the great heat in the dry season. Thus, in the llanos of Venezuela, 
in the warm valleys of California, in parts of India, and in many 
other regions, the surface is covered with verdure during the wet 
season, but assumes the parched aspect of a desert during the dry 
season. 

Deserts. — In still other regions, where the rain-fall is 
very slight, vegetation of any kind is scanty or entirely 
wanting, and a true desert is the result. The animal 



DISTRIBUTION OF LIFE. 33 1 

forms of deserts must manifestly be very different from 
those of forests or meadow lands, not only because 
of the scarcity of water to drink, but because of the 
dearth of vegetable matter which, directly or indirectly, 
affords food to all animals. Hence, desert animals must 
be adapted not only to traverse great distances in order to 
collect enough food from the scanty vegetation, but also 
adapted to escape, by flight, or strength, or concealment, 
from other animals which subsist on animal food. 

The close relation between the rain-fall and the character of veg- 
etation, may be appreciated by comparing the chart of Vegetation 
Regions (page 332), with that of the Rain-fall (page 76). It is seen 
that all deserts correspond to regions of very light rain-fall, and 
that the regions of heaviest forests are in regions of heaviest 
rain-fall. But it is also seen that the northern regions of open for- 
ests in both Ame'rica and Euro-Asia lie mostly in regions of mod- 
erate rain-fall ; while in both South America and Africa, certain 
regions of heavy rain-fall have only meadow-land vegetation. 
These apparent anomalies are the result of inequalities in the 
annual distribution of rain-fall. In low latitudes, the high, uniform 
temperature permits plant growth during the entire year; but where 
wet and dry seasons alternate, the plants are killed to the roots 
during the dry season, and only the low herb or shrub vegetation of 
meadows can mature during the wet season. In high latitudes, 
however, the great annual variation of temperature adapts some 
plants to pass a great part of the year (winter) in a dormant condi- 
tion, and to begin growth in the spring where they left off growing 
in the autumn. In such latitudes, if a small annual rain -fall 
occurs mostly during the short growing season, it produces the 
same effect upon vegetation as a larger annual rain -fall in 
latitudes where the growing season is longer. To a certain alti- 
tude, mountain ranges generally receive a heavier rain-fall than the 
adjacent lowlands, and for this reason are usually forest-clad, though 
they rise from the midst of extensive treeless plains, or even from 
deserts (page 311). 

Distribution of Different Kinds of Life. — While the 
general form and habit of plants and animals are thus 
largely determined by the climate, there are many pe- 




(332) 



DISTRIBUTION OF LIFE. $2>2> 

culiarities which climate alone will not account for, when 
the kinds of plants or of animals in one region are com- 
pared with those in another. If climate were the sole 
cause of the present distribution of plants and animals, 
we should expect to find the organisms of different regions 
having very similar climates more closely related to each 
other than those living in regions having very dissimilar 
climates. But very frequently this is not the case. 

Many of the organisms of semi-tropical Florida and Georgia 
have nearer relatives in the Arctic regions of both America and 
Euro-Asia, than in the closely neighboring Bahama Islands, Cuba, 
or Yucatan, where the climate is so much more nearly the same. 
Parts of equatorial South America and west Africa are almost 
identicalin climate, and yet differ widely in their assemblages of 
plants (flora), and of animals (fauna). The flora and fauna of 
Euro-Asia north of the Himalaya Mountain system differs less from 
that of North America than it does from either that of Africa south 
of the Sahara, or of Asia south of the Himalaya range. These 
latter regions, though often similar in climate, differ greatly from 
each other in flora and fauna, while many of the plants and most of 
the animals of Australia, which closely resembles parts of south 
Africa in climate, have no near relatives in that continent. 

In general, every region of the land that is broadly 
separated from surrounding regions by great differences of 
climate, or strongly marked physical barriers to the pas- 
sage of plants and animals, such as the sea, lofty mountain 
chains, or wide areas of desert, differs to a greater or less 
extent from the surrounding regions in flora and fauna. 
The amount of this difference is roughly proportional (i) 
to the completeness of the separation, or isolation, of the 
regions, and (2) to the length of time they have been 
separated ; — the difference being greatest where the isolation 
is greatest, and has endured for the longest time. 

The development theory explains why this should be the case. 
The environments of organisms in two regions are never exactly 
the same, and each environment is constantly, though perhaps very 




North American Region, 
Euro-Asian Region. 
African Region. 



Australian Region. 
1 : S^. : ': i Transitional regions. 



(334) 



DISTRIBUTION OF LIFE. 335 

slowly, changing, while the organisms and their descendants in each 
region are constantly and gradually changing also — each organism 
adapting itself to its own special environment. Hence, if very 
similar and closely related organisms were placed in two regions 
separated from each other by impassable barriers, their remote de- 
scendants would inevitably be very different, not only from their 
similar ancestors, but from each other ; whereas, if no barriers ex- 
isted, the descendants in either region would constantly mingle with 
those in the other region, and the related organisms would thus tend 
to continue similar in the two regions, though in both they might 
eventually develop features which their ancestors did not possess. 

Primary Biological Regions. — A comparison of the 
floras and the faunas of different lands indicates that the 
continental plateau may be divided into six great biological 
or life regions, each characterized by the abundance of 
certain kinds of plants and animals belonging to great or- 
ganic groups that are not represented at all, or not 
nearly so abundantly, in any other region. Each geo- 
graphical grand division, with one exception, corresponds 
roughly with a single primary biological region. Thus, 
we have (1) the South American region; (2) the North 
American region ; (3) the Euro-Asian region ; (4) the 
African region ; (5) the Australian region ; and (6) the 
Oriental region, which comprises the greater part of the 
Malay Archipelago and the portion of the main-land of 
Asia south and east of the Himalaya Mountains. 

Where the primary regions are not separated by the great 
oceanic depressions, their boundaries generally overlap each other, 
producing transitional regions, in which the characteristic plants 
and animals of both the adjacent primary regions are found to a 
greater or less extent. There are at least four transitional regions : 
(1) the Mexican region, embracing Mexico and the hot desert 
region of the south-west United States ; (2) the Mediterranean re- 
gion, embracing both coasts of that sea and the continuous desert 
territory on the south and east, from the Atlantic to the valley of 
the Indus and the Hindoo Koosh Mountains ; (3) the Chinese region 
from the Nan Ling (mountains) on the south, to the Khin Gan system 
P. G.— 19. 



336 PHYSICAL GEOGRAPHY. 

on the north ; and (4) the Papuan region, embracing Celebes, Papua, 
or New Guinea, and the smaller neighboring islands of the Malay 
Archipelago. On the chart (page 334) the approximate boundaries 
and the relative positions of the primary and transitional regions of 
the continental plateau are indicated. It will be observed that these 
transitional regions occupy territory in which there is a marked tran- 
sition in climate, or well marked peculiarities of surface, such as 
high mountain ranges, broad deserts, or wide areas of the sea, 
which constitute a succession of barriers to the free passage of or- 
ganic forms between the adjacent primary regions. 

A seventh biological region may be said to embrace 
all the oceanic island groups, for although the various 
groups differ from one another in flora and fauna, still they 
all possess in common certain striking peculiarities when 
compared with the continental regions, and these peculi- 
arities throw some light on the general distribution of 
life. 

Characteristics of the Island Region, — Of all regions, 
that of the oceanic islands is most completely isolated, both 
as to space and time. Each island group is not only sep- 
arated from the continents by a great width of ocean, but 
all evidence indicates that it has always been so separated. 
Hence, we should expect that each island group would be 
peopled with species of plants and animals found nowhere 
else in the world, and this, in general, is the case. 
Though peculiar in species, the island organisms almost 
always show a greater or less resemblance in internal 
structure to, and are hence the near or remote kindred of, 
organisms inhabiting the nearest continent — though this 
may be hundreds of miles distant — and this resemblance 
is greatest in islands where strong prevailing winds and 
oceanic currents move directly from the continents toward 
the islands. The bearing of these facts becomes apparent, 
when, upon closer examination, it is found that all the 
native plants and animals of oceanic islands are of kinds 



DISTRIBUTION OF LIFE. 2>27 

which, at some stage of life, are specially adapted for a 
wide dispersal, either through the agency of winds or of 
ocean currents. These facts, in connection with the ab- 
sence of fossil remains of very ancient forms of life, render 
it nearly certain that the oceanic islands are peopled by 
such stray forms of continental plants and animals as find 
a congenial environment after being cast upon their shores 
by the direct or indirect agency of the winds and oceanic 
currents ; and that the amount of peculiarity in any special 
form of island life is proportional to the length of time 
since the arrival of its first island ancestors or any similar 
forms. 

Among the most common plant groups of oceanic islands are the 
ferns, grasses, and sedges ; some of the palms ; the group including 
the thistle and dandelion ; and that to which beans, peas, and the 
locust belong. The seeds of all these plants are lighter than water ; 
they retain vitality long after detachment from the parent stem ; and 
either (i) are very minute and furnished with a wing-like "down," 
which adapts them for transportation by the winds; or (2) are envel- 
oped in a water-proof shell, or pod, which enables them to float safely ; 
or (3) in a bur, which will adhere to the plumage of birds; or (4) are 
so protected in a small fruit or berry, which birds swallow whole, that 
the living seed-germ may be transported in the bird's stomach over 
great distances. There are other groups of plants represented on 
oceanic islands, but seldom or never any having heavy or perisha- 
ble seeds. The native animal groups seem to be exclusively con- 
fined to birds, insects, bats, certain land mollusks, and certain 
reptiles. The first three can fly. Mollusks are very tenacious of 
life ; some can seal their shells up water-tight, and thus float safely 
for many weeks ; others reach islands attached to floating drift- 
wood ; while the living eggs of still others have been found in the 
mud attached to the feet of birds. The eggs of island reptiles, gen- 
erally lizards, are doubtless thus transported, attached to the feet of 
birds or bats. The absence from oceanic islands of all native four- 
footed mammals, all amphibians (toads, frogs, etc.), and all fresh- 
water fishes, renders the fauna strikingly peculiar. The influence 
of the direction of winds and currents upon the amount of pecu- 
liarity of island life is well shown by comparing the life-forms of the 



338 PHYSICAL GEOGRAPHY. 

Bermudas, Azores, and Galapagos islands respectively, with those of 
the nearest continental region, from which they evidently were orig- 
inally peopled. The constant Gulf Stream and regular winter west 
winds so frequently carry American forms of life to the Bermudas, 
that the island breeds are kept similar to the continental breeds, 
and few distinct species are found. The regular winds and currents 
move from the Azores toward Europe, and stray life-forms from 
Europe are brought to the islands only occasionally by the intermit- 
tent cyclonic or storm winds, and hence time is allowed for many 
species to become peculiar. The Galapagos Islands are nearer to 
a continent than either of the other groups, but lie in the calm, 
stormless, equatorial region of the Pacific, and immigrants arrive so 
seldom that all the organisms belong to distinct local species, ex- 
cepting the extensive migrator, the rice-bird, or bobolink. 

No one of the continental regions is so completely 
isolated from all others as the island region, and between 
none of them are the present barriers nearly as permanent 
as the great sea-barrier which isolates the oceanic islands ; 
for, as a result of the constant but gradual upheaval or de- 
pression of portions of the continental plateau, the conti- 
nents and grand divisions have been both more closely 
united, and more effectually separated, than they are at 
present. Climatic barriers have also changed materially 
on the continental plateau, for such alterations of level, 
through their effect upon the direction of oceanic and 
atmospheric currents, have been largely instrumental in 
bringing about such vast changes of climate in the past as 
are evidenced by the occurrence in arctic lands of the 
fossils of tropical plants and animals ; and by the traces of 
continental glaciers, which indicate the existence of an 
arctic climate as far south in the United States as the 
Ohio valley. It has thus been possible in the past for 
representatives of all the great groups of plants and ani- 
mals to migrate to and fro between continental regions 
that are now separated by impassable climatic or 
physical barriers. Hence, these regions differ among 



DISTRIBUTION OF LIFE. 



339 



themselves in organic forms much less than they collec- 
tively do from the island region, and the difficulty of ac- 
counting for the existing differences and resemblances of 
flora and fauna is vastly increased, and frequently rendered 
quite impossible. 

The Australian region is by far the most peculiar of 
the continental regions, since, in addition to a great num- 
ber of peculiar kinds of plants, its mammals, with scarcely 
an exception, belong to two small and very peculiar sub- 
groups (Fig. 140),— the monotremes, or egg-layers, which 
are found in no other region, and the marsupials, or 




rag 



Buck-Mole. 



Brush-turkey. Emu. 



Fig. 140. — Characteristic Animals of Australia. 

pouched animals. These, though represented by the 
kangaroo and many variously adapted forms in Australia, 
are not represented by living varieties in any other region 
excepting the two most distant from Australia : — a few 
kinds of this subgroup (about twenty varieties of opossum) 
being found in South America, and two varieties occurring 
in the United States. 

Among the more characteristic and peculiar plants of Australia 
are the leafless beef-wood trees, the very numerous and generally 
leafless varieties of the acacia; the great eucalyptus trees, whose 
leaves grow with their edges to the sky, so that they cast but little 
shade ; the heather-like epacris ; and (especially in New Zealand) 
many filmy- and tree-ferns. The characteristic plants are most nu- 
merous in southern Australia, while in the north they are mixed 



340 



PHYSICAL GEOGRAPHY. 



with palms and other tropical plants, identical or nearly so with the 
plants of the Malay Archipelago and south-eastern Asia, from which 
region they have doubtless reached Australia, in comparatively 
recent times. The peculiar birds of Australia include piping crows, 
honey-suckers, lyre-birds, cockatoos, gayly colored pigeons, brush- 
turkeys or mound-builders, and the almost wingless emus and cas- 
sowaries, whose nearest relatives are the ostriches of Africa and the 
rheas of South America. 




Sloth. 

Rhea. 

Prehensile-tailed Monkey 



Fig. 141. — Characteristic Animals of South America. 



The South American region, though its flora and 
fauna are among the richest and most varied in the world, 
probably ranks after the Australian as the most peculiar 
region. The striking characteristic of this region is the 
preponderance in its fauna of lowly organized types (Fig. 



DISTRIBUTION OF LIFE. 34 1 

141). The marsupials (opossums), edentates (sloths, ar- 
madillos, and ant-eaters), and such rodents as the cavys, 
guinea-pigs, and agoutis, form the majority of the mam- 
mals, while the more highly organized carnivora and 
hoofed-animals are not only exceedingly deficient, but are 
smaller than their kindred in the old world ; the tiger, 
lion, rhinoceros, camel, and hog of the old world, being 
represented by the respectively smaller jaguar, puma, 
tapir, llama, and peccary in South America. The same 
relatively low type of organization characterizes the South 
American monkeys and birds. The nostrils of the former 
face outward instead of downward, and most of them 
are prehensile-tailed; and an unusually large proportion of 
the birds are songless; while the tinamous and rhea, and 
the curassow, are allied respectively to the lowly organized 
ostriches, and to the brush-turkeys of Australia. 

Among the hundreds of peculiar plants of the South American 
region may be mentioned the mahogany, rose-wood, and logwood, 
the cinchona or Peruvian bark tree, and plants yielding India rubber, 
many spices, balsams, and varnishes, a great variety of laurels, 
bean-trees, and palms, many cacti and orchids or air-plants (one of 
the latter yielding the vanilla bean), peculiar varieties of bananas, 
tree-grasses (bamboo), and tree-ferns, and the peculiar varieties of 
the nightshade family, such as Cayenne pepper, the potato, and to- 
bacco, while Indian corn and the tomato are probably descendants 
of plants native in this region. 

The African and Oriental regions possess marked 
peculiarities by which each may be distinguished from all 
the other regions; but the flora and fauna are more com- 
plicated, highly organized forms of life are more numer- 
ous, and the difficulty of accounting for the present 
distribution is vastly greater than in the two preceding 
regions. The African region is the more peculiar. It 
differs markedly from South America and Australia in 
the great development of the highly organized carnivora 



342 



PHYSICAL GEOGRAPHY. 




Aard-vark. 


Pangolin. 


Ostrich. 


Giraffe. 


Gnu. 


Zebra. 


Lion. 


Hippopotamus. 


Gorilla 



Fig. 142.— Characteristic Animals of Africa. 

and hoofed-animals, especially antelopes, of which more 
kinds (80 or 90) are found than in any other region. Be- 
sides these, the hippopotamus, giraffe, zebra, quagga, and 
wild ass (the ancestor of the domestic animal) are found 
nowhere else. Such widely spread animals, however, as 
bears, moles, deer, sheep, and goats are completely absent 
from the African region. With these marked differences, 
this region more than any other resembles South America 
in possessing a moderate number of the lowly organized 
edentate animals (the ant-eating aard-varks and pango- 
lins). The carnivorous animals include the lion, leopard, 
panther, several kinds of hyenas, the jackal, aard wolf, and 



DISTRIBUTION OF LIFE. 



343 




Jungle-Bear. 

Gayal. 
Orang-outang. 



Tiger. 

Mzintjac. 

Silver Pheasant. 



Tapir. 

Chevrolain. 

Peacock Pheasant. 



Fig. 143. — Characteristic Animals of the Oriental Region. 

a great variety of civet cats. Most of these, as well as near 
relatives of the African elephant and rhinoceros, are also 
found in south-eastern Asia, and are characteristic of the Ori- 
ental, rather than of the African, region. The monkey tribe 
of the two regions embraces the man-like apes — the gorilla 
and chimpanzee of Africa, and the orang-outang of Farther 
India, — besides numerous baboons, true monkeys, and the 
peculiar half monkeys, or lemurs. Among the peculiar 
African birds may be mentioned the true ostrich, the 
serpent-eating secretary-bird, many guinea-fowls, vulture- 
crows, plaintain-eaters, crested tourocos,and colies. 

The Oriental region is characterized by its orang- 
outangs, and by a greater development of carnivorous 



344 PHYSICAL GEOGRAPHY. 

animals than Africa; for, in addition to the lion, leopard, 
hyena, etc., it possesses the tiger and ounce. It differs 
from Africa, too, in having bears, several kinds of deer 
(muntjac, chevrotain, etc.), wild cattle (the gayal and 
others), wild hogs, and a tapir closely related to the 
South American animal. This region is the head-quarters 
of true mice and squirrels, and contains some very peculiar 
flying lemurs. Among its characteristic birds are babbling- 
thrushes, hill-tits, green bulbuls, numerous crows and horn- 
bills, and a great variety of magnificent pheasants, including 
the peacock and jungle-fowl, from which the domestic 
chickens are descended. 

The African flora embraces the oil-palm, the great baobab, eu- 
phorbias, bignonias, and tamarinds, together with many varieties of 
laurel, fig, myrtle, acacia, and mimosa in the dense forest region of 
the west. On the more open eastern tablelands, tall grasses, sedges, 
and the coffee tree are characteristic plants ; while the remarkably 
rich flora of the south includes a great variety of heather, fig- 
marigolds, and aloes, together with some close allies of characteristic 
Australian and South American plants. The flora of the Oriental 
region as a whole has fewer distinctive features than that of Africa 
or South America. The region is relatively small, and has a great 
climatic range, from the perpetual snows of the Himalayas to the 
equatorial lowlands of Java, and hence many plants from adjacent re- 
gions can reach congenial environments within its boundaries. Along 
the Himalayas many pines, junipers, yews, cedars, and some oaks 
occur. In the dry districts of north-west India many acacias and 
the tamarisk are found. In the moist forest regions of the south 
and south-east, pitcher-plants, wood-oil trees, custard-apples, man- 
goes, numerous palms, cycads, and many spice-yielding plants 
abound, while teak, toon, sal, ebony, satin-wood, sandal-wood, and 
iron-wood are characteristic timber trees. 

The Euro-Asian and North American regions, 

though more extensive, differ from each other less in flora 
and fauna than any other two regions. By some they are 
classed as a single region, but minor differences seem to 
warrant their division. The more common and conspicu- 



DISTRIBUTION OF LIFE. 



345 



ous animals, such as the various wild cats, lynxes, wolves, 
foxes, weasels, bears, elk, deer, voles, beavers, squirrels, 
marmots, and hares, are identical, or very similar, in the two 
regions. Even the grizzly bear of the Rocky Mountains 
and the buffalo (bison) of the Great Plains are thought to 
be but slight variations of the European brown bear and 
the nearly extinct aurochs of west Russia. But with 
these there are in each region numerous less conspicuous 
animals not found in the other, and some found no- 
where else ; thus, the star-nosed mole, skunk, raccoon, 




Chamois. 

Hedgehog. 

Fig. 144. — Characteristic Animals of Euro-Asia 



Wild Boar. 
Desman. 



puma or panther, prong-horned antelope, Rocky Mountain 
goat, big horn sheep, musk-ox, musk-rat, jumping-mouse, 
prairie-dog, gopher, tree-porcupine, sewellel, otter, and 
opossum occur in North America but not in Euro-Asia, 
while hedgehogs, wild boars, dormice, badgers, camels, 
yaks, saiga antelopes, and nineteen kinds of wild sheep 
and goats occur in Euro-Asia but not in America. 

The same general resemblance with numerous special differences 
characterizes the birds and plants of the two regions. Eagles, owls, 



346 



PHYSICAL GEOGRAPHY. 




Big Horn. 
Prairie-dog. 



Prong-homed Antelope. 
Opossum. 



Fig. 145. — Characteristic Animals of North America. 

hawks, crows, thrushes, wrens, tits, and finches occur in both, but 
America alone possesses humming-birds, wild turkeys, turkey-buz- 
zards, blue jays, tanagers, hang-nests, and mocking-birds ; while 
Euro-Asia is peculiar in its starlings, magpies, nightingales, true fly- 
catchers, partridges, pheasants, vultures, etc. Among plants, both 
regions exceed all others in the development of the pine family, includ- 
ing the pines, larches, spruces, firs, hemlocks, cedars, etc.; and of the 
oak family, including oaks, chestnuts, beeches, hornbeams, etc. 
The ash, elm, sycamore, walnut, and maple are also charac- 
teristic plants, as are also numerous gentians, rushes, primroses, 
birches, willows, and saxifrages. The American region is peculiar 
in its asters, golden-rod, sequoias, and bald cypresses, while Euro- 
Asia shows the greater development of heather, roses, olives, 
almonds, etc. 

Taken as a whole, the more highly organized forms 
of life preponderate on the land masses of the northern 
hemisphere — North America and Euro- Asia, — the great 
central region of the continental plateau (see chart, pages 
1 S 2 > I 53); while forms of land life of lower organization 
are characteristic of the extremities of the continental 



DISTRIBUTION OF LIFE. 347 

plateau which project into the southern hemisphere — 
South America, south Africa, and Australia. Now, when 
all the fossils of ancient organisms from different parts of 
the world are compared, it is found that the collection 
from the three northern regions includes not only the evi- 
dent ancestors of the forms now occupying those regions, 
and of the more highly organized forms now confined to the 
southern hemisphere, but from the older rocks come the 
fossil ancestors of the lowly organized forms now charac- 
teristic of the southern land masses. 

Not only do the fossils prove that the elephant, rhinoceros, and 
hippopotamus were once far more abundant in Europe than they 
are now in the tropics, but they also prove that the man-like apes 
of west Africa and Malaya, the lemurs of Madagascar, the edentata 
of South America and Africa, and the marsupials of Australia and 
South America were all inhabitants of Euro-Asia and North America 
at the beginning of the tertiary era of geological time. Though 
animal forms are preserved as fossils much more frequently than 
vegetable, still the indications are that the same facts are as true of 
plants as of animals. 

These facts indicate that at least during the vast 
length of time that has elapsed since the beginning of the 
tertiary era, and probably for much longer, the great land 
masses of the northern hemisphere have occupied substan- 
tially their present sites, and that in this great compact 
central region of the continental plateau, as it underwent 
the long series of changes which resulted in its present 
physical conditions, all the successive types of land organ- 
isms gradually developed, from the lowest to the highest. 

In the southern hemisphere, there appear to have 
been three smaller but equally ancient land masses, vary- 
ing gradually in extent, but always keeping distinct from 
each other, and occupying roughly the sites of Australia, 
South America, and south Africa, respectively. From time 
to time gradual movements of the earth's crust tempo- 



348 PHYSICAL GEOGRAPHY. 

rarily united these isolated extremities of the continental 
plateau with the great central land mass, and during each 
connection, which may have lasted thousands of years, 
the various forms of life then prevalent in the central con- 
tinents gradually found their way southward. After a 
longer or shorter union, the gradual subsidence and sub- 
mergence of the connecting land stopped this migration, 
and a period of isolation began. 

During these periods of isolation, the organisms in 
the central region developed much more rapidly than their 
respective kindred in the detached region, not only be- 
cause the central region underwent more frequent changes 
of environment, through movements of the earth's crust, 
erosion, etc., within its more extensive area, but because 
it contained a greater variety of environments, and hence 
developed a greater variety and a greater number of or- 
ganisms. Therefore, competition for food and other 
means of existence was very sharp in the central region; 
each organism was compelled to use all its faculties to se- 
cure a livelihood, and all but the more perfectly adapted 
organisms perished. Thus the more lowly organized 
forms of life gradually became extinct, and were succes- 
sively replaced by their more . highly organized descend- 
ants. In the relatively small isolated regions, however, 
the number of organisms was relatively small, and the 
competition for food was not sharp ; and in consequence 
the descendants of the lowly organized immigrants, 
though changing slightly, developed toward a higher or- 
ganization with extreme slowness. Thus, upon the re- 
union of an isolated region with the central land, the 
migrants from the latter were much more highly organized 
than the inhabitants of the newly attached region ; and if 
the immigrants were numerous, they gradually exterminated 
and replaced their competitors of lower organization. 



DISTRIBUTION OF LIFE. 349 

Australia appears to have had but one such union with the central 
region, and that at a very early period, when monotremes and mar- 
supials were the predominant forms of mammalian life. South 
Africa and South America each appear to have had a succession of 
such unions and separations, allowing the immigration first, of low 
forms only (edentates, lemurs, etc.); subsequently, of rodents and 
small carnivora ; and lastly, of the higher forms of apes, car- 
nivora, and hoofed-animals. It appears that North America and 
Euro-Asia have frequently and for long periods been more closely 
united in the arctic regions than they are to-day, and at times when 
a moderate polar climate permitted an easy interchange of organic 
forms ; yet there probably was a time in the remote past when the 
arctic separation was more complete than at present, and when 
North America was a relatively small, isolated region of the great 
continent of Euro-Asia, which is thus probably the remote source 
from which all other regions were supplied with their higher forms 
of life. At that time, it is probable the Oriental and Euro-Asian 
regions were one — their final separation dating from the great up- 
heaval of the Himalaya Mountain system. 

Of the distribution of marine life, and the laws 
which govern it, but very little is known. The sea being 
continuous, and the water below a comparatively slight 
depth having an almost uniform temperature, it is not 
surprising that life-forms in various regions of the sea do 
not differ so greatly as in the various regions of the land. 
Nevertheless there are differences, the reasons for which 
are still in the highest degree problematical ; thus, the 
king crab is found only on the widely separated coasts of 
Nova Scotia, Japan, and the Malay Archipelago. Marine 
vegetable life, with the possible exception of microscopic 
diatoms, seems to be confined to a depth of less than ioo 
fathoms, which is about the depth to which the luminous 
rays of the sun can penetrate the water. Marine animal 
life, however, though more abundant near the surface, 
exists near the bottom of all parts of the oceans, where, 
possibly on account of the greater food supply, it is 
thought to be more abundant than at intermediate depths. 



CHAPTER XXIV. 



MAN. 



Lord, thou deliveredst unto me five talents : behold, I have gained beside them 
five talents more. His lord said unto him, Well done, thou good and faithful 
servant : thou hast been faithftil over a few lhi?tgs, I will make thee ruler over 
many things. — Matthew xxv : 20, 21. 

Man. — One of the most eminent naturalists and stu- 
dents of mankind has said : ' ' The organized world pre- 
sents no contrasts and resemblances more remarkable than 
those which we discover on comparing mankind with the 
inferior tribes. That creatures should exist so nearly ap- 
proaching to each other in all the particulars of their 
physical structure, and yet differing so immeasurably in 
their endowments and capabilities, would be a fact hard 
to believe if it were not manifest to our observation. In 
all the principles of his internal structure, in the compo- 
sition and functions of his parts, man is but an animal. 
The lord of the earth, who contemplates the eternal order 
of the universe, and aspires to communion with its invisi- 
ble Maker, is a being composed of the same materials and 
framed on the same principles as the creatures which he 
has tamed to be the servile instruments of his will, or 
slays for his daily food." 

The fundamental resemblance of man to all animals 
without exception, from the most highly organized man- 
like ape down to the single-celled amoeba, is that he and 
they alike require organic, protein food. Man resembles 
the vertebrate animals much more closely than any others, 
(350) 



MAN. 



351 




P. G— 20. 



since he and they possess 
an internal bony skeleton, 
built upon a jointed back- 
bone, containing a spinal 
nerve-cord, which leads from 
a principal nerve-center, or 
brain, in the head. Still 
closer is his likeness to the 
mammals, which, like man, 
nourish their young from 
the breast. 

While possessing a close 
structural resemblance to all 
mammals, man resembles some 
kinds much more closely than 
others, as indicated in Fig. 146. 
The resemblance between man 
and the man-like apes is indeed 
much closer than may be appre- 
ciated from this sketch, for even 
such details of structure as de- 
termine the difference between 
the hand and the foot of man 
are found to be also well marked 
in the higher members of the 
monkey tribe, which therefore 
possess true hands and feet. In 
fact, the extremities of the man- 
like apes differ in structure less 
from those of men than from 
those of the lowest monkeys, or 
marmosets (Fig. 147). 

Man differs most widely 

from all the lower animals 
in his vastly greater mental 
capabilities. The organ of 
the mind — the brain — is, 



352 



PHYSICAL GEOGRAPHY. 




hand CHIMPANZEE foot 




hand GORILLA foot 



HAND MARMOSET. FOOT 



Fig. 147.— Hand and Foot of Man and Monkeys. 



on the average, about three times as large in man as in 
those animals which resemble him most closely in struct- 
ure; — the average man having about 87 cubic inches of 
brain, while the gorilla, an animal about twice as heavy 
as man, has less than 30 cubic inches. In addition to 
this, the surface of man's brain is furrowed by a vastly 
more complicated system of fissures, or sulci, and inter- 
vening folds, or convolutions. The surface area of man's 
brain is thus greatly enlarged, and mental power is sup- 
posed to depend to a great extent upon the surface area 
of the brain (Fig. 148). 

Though the brain is larger and mental power immeasurably- 
greater in man than in the higher beasts, the difference in the 
structure of the brain is relatively slight. Prof. Huxley, one of the 
greatest anatomists of the present age, has carefully compared the 
brain of man with that of various members of the monkey tribe, 
and has found that " so far as cerebral (brain) structure goes, man 
differs less from the chimpanzee or orang-outang than these do from 



MAN. 



353 



the (lower) monkeys," while "the (structural) difference between 
the brains of the chimpanzee and of man is almost insignificant, 
when compared with that between the chimpanzee brain and that of 
the lemur (the peculiar half-monkey of Madagascar). The argu- 
ment," Huxley continues, "that because there is an immense differ- 
ence between a man's intelligence and an ape's, therefore there 
must be an equally immense difference between their brains, appears 
to me about as well based as the reasoning by which one would 
endeavor to prove, that because there is a 'great gulf between a 
watch that keeps accurate time and another that will not go at all, 
there is therefore a great structural hiatus (difference) between the 




LEMUR 



ORANG 



FJs;. 148. — Brain of Man and Monkeys. 



two watches. A hair in the balance wheel, a little rust on a pinion, 
a bend in a tooth of the escapement, a something so slight that only 
the practiced eye of the watchmaker can discover it, may be the 
source of all the difference. And believing, as I do (with Cuvier), 
that the possession of articulate speech is the grand distinctive 
character of man, I find it easy to comprehend that some equally 
inconspicuous structural difference may' have been the primary 
cause of the immeasurable and practically infinite divergence of the 
human from the ape (family)." 

The fact that, physically considered, man resembles 
the higher beasts as closely as these resemble the lower 



354 PHYSICAL GEOGRAPHY. 

animals, in connection with the fact, now generally ad- 
mitted, that the higher and lower animals are but differ- 
ently modified descendants of a common kind of ancestors, 
leads to the inference, that man himself is a still differently 
modified descendant of the same remote ancestors. The 
difference in the degree of mental power, however, be- 
tween civilized man and even the highest beast, is appar- 
ently so much greater than that between the highest and 
even the very lowest animal, that it is hard to conceive of 
any natural process, by which such an almost infinite diver- 
gence could have taken place in organisms descended, 
however remotely, from similar ancestors. 

Whatever may have been the origin of man, and 
whatever may be his true relationship to the higher ani- 
mals which he resembles so closely in structure, we know 
that he has been an inhabitant of the earth for a time 
very much greater than the 4,000 or 5,000 years of which 
history or tradition preserves a record. This is known 
from the occurrence of fossils of man — human bones, and 
implements made by man — associated in deposits with the 
fossils of extinct animals of the late tertiary or early qua- 
ternary era. This, though quite recent when compared 
with the lapse of geological time, indicates that man, as a 
tool- or implement-making animal, inhabited the earth 
tens, or possibly hundreds, of thousands of years ago, 
while relics of man, found in deposits of intermediate ages, 
indicate that he has inhabited the earth continuously dur- 
ing this long but indefinite period. 

We know that man has changed but little in his 
structure and manner of thought during the period cov- 
ered by history and tradition, but that his general knowl- 
edge and intelligence have increased greatly during this 
time. Although, at the dawn of history or tradition, 
man in ancient Egypt dwelt under an organized govern- 



man. 355 

ment, knew the use of the more useful metals, practiced 
the art of agriculture, had sufficient knowledge of me- 
chanics to build monuments which have endured until the 
present, and was thus vastly more civilized than many 
savage tribes are to-day ; still, with all these attainments, 
his civilization was very greatly inferior to that of the 
present time. In following the history of mankind from 
that day to this, one may note the more or less gradual 
increase of knowledge, its broader diffusion among the 
masses, and the consequent slow, but on the whole con- 
tinuous, progress in general intelligence and civilization. 
The same kind of progress in the intelligence of pre- 
historic man may be dimly traced, by comparing his 
implements and other indications of his work and habits, 
as they are occasionally found in deposits of successively 
older date. As the age of the deposit increases, the im- 
plements become less various in shape, and simpler and 
ruder in construction ; while the associated remains, when 
they afford any indication of the manner of life of pre- 
historic man, indicate that this was simpler and ruder in 
proportion as the deposit is older. 

Thus, in going backward from the beginning of historic times in 
various parts of Europe, we find that the more modern prehistoric 
races of portions of that region made implements of iron and 
bronze, as well as of stone ; had domesticated such animals as the 
ox, dog, sheep, and goat; lived together in settlements, and culti- 
vated wheat, and the same kind of barley found wrapped with old 
Egyptian mummies. In earlier deposits, only implements of bronze 
and stone are found, while the associated bones of the dog, horse, and 
ox, and other remains indicate a pastoral rather than an agricultural 
life. From still older deposits come only implements of stone or 
bone, while the dog seems to be the only domestic animal. These 
stone implements, however, are neatly and symmetrically shaped, 
and have generally been ground down to a smooth and often polished 
surface, indicating a degree of intelligence in the makers decidedly 
greater than that found in the rudest savages. The oldest of all 



356 PHYSICAL GEOCxRAPHY. 

implements of prehistoric man are those found associated with 
fossils of animals now extinct. These earliest implements are also 
of stone, but are much more rudely made than the later ones, being 
simply flakes of flint, or other hard stone, roughly chipped into un- 
symmetrical shapes to obtain a cutting edge or a sharp point. Such 
implements are to-day made and used by the lowest and most 
ignorant tribes of savages, but are found in more or less deeply 
buried deposits in all parts of the world, as the most ancient 
vestiges of man. 

Such facts as these are held to indicate that all 
men — the most cultivated races as well as the rudest — 
have descended from more or less remote ancestors who 
were as ignorant, and as low in the scale of intelligence 
and civilization, as the lowest savages of whom we have 
any knowledge. During the vast period of time which 
has elapsed since all mankind was at this low state, dif- 
ferent portions of the human family have developed their 
mental powers at different rates, resulting in the various 
degrees of intelligence and civilization found among the 
people now inhabiting different regions of the globe. 

It is to be remarked that though the indications of this progres- 
sive development of the human intelligence are so strong and nu- 
merous as to render the fact practically certain, still the most ancient 
traces of man yet found indicate a being no less distinctly human 
than are the lowest savages of Borneo and Tierra del Fuego to-day. 
But the gap separating the intelligence of a naked savage of the 
Borneo forest from that of an Emerson, a Spencer, a Gladstone, or 
a Bismarck, is very great, and the daily accumulating evidence that 
the latter has developed by gradual modification from the former 
has suggested to some that the low intelligence of the savage may 
itself have developed, by a similar process, during the long ages of 
the remote past, from a still lower state, in' which it was similar to, 
or identical with, what we call instinct in animals. 

Certain superficial differences in physical features 
are found to distinguish men who for long periods have 
lived in separated regions. Because these differences are 
superficial, they are often quite conspicuous ; but when 



man. 357 

the deeper seated and more essential structural features 
are compared, they are found to be so wonderfully sim- 
ilar in men from every region, as to warrant the belief that 
all mankind is descended from a single race. 

An Englishman can generally be distinguished from a Dutch- 
man by indefinable peculiarities of physical feature; but it is well 
known that the ancestors of these peoples formed a single race, 
and lived together in the region between Denmark and Belgium. 
About the year 500, a portion of this people went over and con- 
quered Britain, where they settled and continued to dwell. These 
emigrants did not differ materially in feature from their former 
neighbors who remained at home, but their descendants were in 
great part isolated from their kindred on the main-land, and each 
portion, by adaptation to its special environment, gradually devel- 
oped the peculiar features which characterize either people to-day. 
The fact that the slight but plainly perceptible race differences be- 
tween the English and the Dutch have thus developed during 1,400 
years of imperfect isolation of the two peoples, in two regions so 
closely adjacent as to have nearly the same climate and general 
surroundings, is held to afford conclusive evidence that the greatest 
differences between the most divergent races of men, may be ac- 
counted for by the operation of the same processes, during the 
vastly longer period of man's occupancy of the earth, and on de- 
scendants of an originally similar people, who became completely 
isolated from each other, in regions so widely separated as to differ 
markedly in climate and other conditions of environment. 

Resemblances in language and customs are often 
found in races which now occupy widely separated regions 
and differ markedly from each other in physical feature. 
While such resemblances can not be found between all 
languages, they are thought, when they do occur, to 
afford direct evidence that these particular languages are 
but more or less divergent variations from a single prim- 
itive tongue, and that the races using them are descend- 
ants of the single race that used the primitive language. 

Indeed, language is thought to afford the best available means 
for tracing the connection between various races. If the resemblance 



358 PHYSICAL GEOGRAPHY. 

is strong, involving whole sentences or very many words, the people 
are supposed to have been separated for a relatively short time. If 
the resemblance is only in an occasional word, the separation of the 
languages and the people using them, from the parent stock, is thought 
to be of very ancient date. If no resemblance at all can be traced 
between languages, the separation of the people using them from 
a common ancestral race is thought to have occurred at an exceed- 
ingly remote period, possibly before the race had acquired a commoi. 
language. 

The inevitable tendency in any people to change, 
which accompanies broad distribution and the consequent 
variety in environment, implies that the race from which 
the whole human family is thought to have descended, 
originally occupied a region of rather limited extent, and 
that the world was peopled by the descendants of various 
portions of this race, who gradually wandered from their 
ancestral home in different directions. 

While there is thus some reason for supposing that man overran 
the earth at an immensely remote period, from some rather small 
central region, it is utterly impossible, in the present state of knowl- 
edge, to locate this region with any degree of accuracy. The fact 
that the regions inhabited by the three most widely divergent types 
of mankind at the present time, approach each other most closely 
in southern and south-western -Asia, is held by many to indicate that 
the ancestral home of man was in that region ; and the fact that all 
of the higher animals seem to have had their earliest development 
in the great land mass of the northern hemisphere, may be said to 
favor the view that man, the highest of all organisms, was not an 
exception to this rule. 

Classification of Mankind. — Varieties of men are 
usually distinguished by differences in the character of the 
hair, formation of the language, color of the skin, and 
shape of the skull. The formation of the hair, and to a 
lesser extent the color of the skin, seem to be more 
strictly hereditary than the form of the skull; and from 
more or less conspicuous differences in these features, all 
mankind may be divided into three broad classes, or types : 



MAN. 



359 




Woolly-haired. Straight-haired. Wavy-haired. 

Fig. 149.— The Three Types of Mankind. 

(1) the woolly-haired and brown-skinned type; (2) the 
straight-haired and yellowish-skinned type ; and (3) the 
wavy-haired and whitish-skinned type. 

These types differ from each other entirely in the formation of 
the languages used, and each type includes several groups, or races, 
which resemble each other in the more general type-characteristics, 
but generally differ widely in language and in minor details of feat- 
ure, while each race is subdivided chiefly by minor differences of 
language into smaller groups, or tribes. The different tribes, races, 
and types, however, graduate insensibly into each other from long- 
continued cross marriages between different peoples, so that it is 
often impossible to draw hard and fast lines between them. 

The woolly-haired type is characterized by its woolly 
or kinkled hair, and by the brownish color of the skin, 
which ranges from almost black to a light brownish tint. 
The peculiar character of the hair results from the fact 
that each hair, when duly magnified, is found to be flat, 
or tape-like. As a rule, the head in this type is very long 
from front to back in proportion to its width, and the 
jaws generally project forward, giving the profile of the 
face a backward slant from the mouth to the low, reced- 
ing forehead. This peculiarity is stronger in some tribes 
than in others ; it is still stronger in the monkey tribe, 
and is most strongly marked in the quadrupeds. The 



360 PHYSICAL GEOGRAPHY. 

mental development of this type as a whole is lower than 
that of the other types. No native woolly-haired race 
has ever had a written history. All races of this type 
are native in the southern hemisphere, which is thus char- 
acterized in its human, as well as in its animal inhabitants, 
by a relatively low state of development. 

There are four principal races of the woolly-haired type. (1) The 
Papuan race inhabits the islands from New Guinea east to the 
Fijis, the mountainous interior of the Malay peninsula, Borneo, the 
Philippines, and several islands of the Pacific. This race has but 
lately become extinct in Tasmania. This race is the lowest of the 
type, is nearly black, with thick, protruding lips and kinkled hair, 
growing in separate tufts over the head. (2) The Hottentots are 
now confined to the southern part of Africa (Namaqua Land and 
the interior of Cape Colony), and are rapidly approaching extinction. 
Though the skin is a yellowish-brown, this race resembles the 
Papuan in having a very flat face, thick, protruding lips, and hair 
growing in separate tufts. (3) The Kaffres inhabit the rest of South 
Africa. To this race belong the Zulu, Zambezi, and Mozambique 
tribes on the east, the great Bechuana peoples in the interior, and 
the Herrero and Kongo tribes on the west coast. Unlike the two 
preceding races, the woolly hair of the Kaffre race grows as a con- 
tinuous fleece over the head. The race has a high forehead, a 
prominent nose, and but slightly protruding lips. (4) The Negro 
race inhabits the Soudan, and the southern part of the Sahara from 
the upper course of the Nile to the Atlantic. The skin is very dark 
brown, and velvety to the touch; the woolly hair, like the Kaffre's, 
grows as a fleece, but the forehead is flatter and lower, the nose 
broad and thick, and the lips large and protruding. 

The straight-haired type of mankind is characterized 
by its coarse, straight black hair, each hair being cylin- 
drical, — that is, having a circular section. The color of 
the skin varies from brown through yellow to a reddish, 
but generally a yellowish tone is present. Many of the 
races of this type are round headed, the length and width 
of the skull when seen from above being nearly equal. 
The forehead is generally less receding, the jaws less pro- 



MAN. 361 

tuberant, and the mental development is higher as a rule 
than in the woolly-haired type. One race, however — the 
Australian — is classed with this type on account of its 
straight, coarse hair ; but it has the dark color, slanting 
face, and protruding lips of the woolly-haired type, and is 
considered to represent one of the lowest states, if not the 
lowest state, of mental development in living man. 

There are five races of this type. (1) The Australian race is 
confined to the main-land of Australia. The mental and physical 
development of this race is very low, the bones being remark- 
ably weak and delicate in structure. It is a significant fact that 
Australia, which is thus occupied by probably the least highly or- 
ganized race of men, should also be strongly characterized by the 
lowest of the mammals — the monotremes and the marsupials. (2) 
The Malay race, though not numerous, is very widely distributed, 
embracing, the bulk of the people in the Malay peninsula, the 
Malay Archipelago, and most of the islands of the Pacific and 
Indian oceans, from the Hawaiian Islands on the east to New 
Zealand and Madagascar on the west. (3) The Mongolian forms one 
of the most numerous races on the earth, embracing in its many 
tribes almost all the inhabitants of Asia from Okhotsk Sea to the 
Bay of Bengal, and westward north of the Himalaya Mountains into 
eastern Europe, while the Lapps and Finns of Scandinavia, the 
Volga Finns of central Russia, the Magyars of Hungary, and the 
Turks of the Balkan peninsula and Asia Minor are isolated tribes of 
this race. The many languages spoken by different peoples of this 
race may be divided into two groups, which are very remotely con- 
nected. The skin of the race is always yellowish in tone, but varies 
in different tribes from a dark brownish-yellow to a light greenish- 
yellow. The face is generally round, with prominent cheek bones, 
while the eye-openings are narrow, and generally slant downward 
toward the nose. (4) The Esqnimos inhabit Kamchatka and the 
north-east extremity of Asia, the Arctic Archipelago, and a narrow 
strip of the Arctic American coast from Alaska peninsula to New- 
foundland. The Esquimos are short, of stout build, with the round 
face and slanting eyes of the Mongolians, and a brownish skin, 
toned with yellow or yellowish-red. (5) The Americans, or Red-skins, 
occupy both North and South America. Many extremely different 
languages prevail in this wide extent of territory, yet all may be 



362 PHYSICAL GEOGRAPHY. 

referred to a single primitive tongue. This race seems most closely 
related to the Esquimo and Mongolian. It is characterized by a 
medium-shaped head neither long nor round, straight, black hair; 
broad, low forehead ; prominent nose and cheek bones, thin lips, 
and a brownish skin, strongly tinged with red or reddish-yellow. 

The wavy-haired type of the human family is distin- 
guished by hair much softer than that of either of the 
other types. It is neither lank nor kinkled, but usually is 
inclined to be wavy whenever allowed to grow long. The 
section of each hair is elliptical in shape. In this type the 
beard grows much more freely and thickly than in either 
of the others. The face is oval in shape, the forehead 
high and prominent, and the jaws do not protrude ; hence, 
the general profile of the face is nearly vertical. The 
color of the skin varies, as in the other types, but in the 
vast majority of cases is so much lighter as to be called 
white in comparison ; but it is usually tinged with pink, 
and in some instances is a dark brown. The type includes 
races which vary widely in mental development, but as a 
whole it may be said to have reached a decidedly more 
advanced state than any other type. 

There are three races of the wavy-haired type. (1) The Dravidicm 
race is confined to Ceylon and the plateau of Deccan. This race 
is exceedingly difficult to classify. It has the wavy hair and strong 
beard of this type, and the language bears a resemblance to that of 
some of the Mediterranean tribes, but the skin varies from a light 
yellowish-brown to a very dark brown. The face is oval, the fore- 
head high, nose narrow and prominent, but the lips are slightly 
protruding. (2) The Nubian race is also hard to classify. It in- 
habits Nubia, Kordofan, and Dongola, and various tribes of the 
northern Soudan seem to carry it westward nearly to the Atlantic. 
The hair is brown or black, and wavy — not woolly — and the lan- 
guage has no resemblance to that of any negro tribe. The beard is 
well developed, the face oval, forehead high, and nose prominent, 
but the skin is a dark yellowish- or reddish-brown. (3) The Cau- 
casian or Mediterranean race is the most highly developed of the 



MAN., 363 

races. It formerly inhabited a region extending from the Bay of 
Bengal west to the Atlantic, embracing south-west Asia, northern 
Africa, and nearly the whole of Europe ; but during the last 500 
years, representatives of this race have spread over nearly the whole 
globe. This race is the most numerous on earth, and, with the 
Mongolian race, is the only portion of mankind possessing a writ- 
ten history. The hundreds of different languages and dialects of 
the modern descendants of the Mediterranean race may be traced 
to four distinct primitive languages, and upon this the classification 
of the race into four main branches is chiefly based ; viz., (a) The 
Basques, who formerly occupied the whole of south-western Europe, 
but are now confined to a narrow region on the northern coast of 
Spain, (b) The Caucasians, confined to a small territory between 
the Black and Caspian seas about the Caucasus Mountains, (c) The 
Semitic branch, including in one group the Berbers of the Sahara 
west to the Atlantic, the Ethiopian, Galla, Somali, and other tribes 
on the north-east coast to the equator ; and in another group the 
Jews, Syrians, ancient Chaldeans, and Arabs, a tribe of the latter 
forming the inhabitants of Abyssinia, (d) The Indo-Germanic or 
Aryan branch, which includes the ancestors of the Hindoos and Per- 
sians ; the Grseco-Romans, ancestors of the Greeks, Albanians, and 
Italians; the Celts, ancestors of the ancient Gauls, the Irish, and 
Welsh ; the Slavonians, ancestors of the Russians and Bulgarians ; 
and the ancient Germans, or ancestors of the modern Germans, 
Dutch, Scandinavians, Anglo-Saxons or Englishmen, and of a vast 
majority of the present inhabitants of the United States. 

The present population of the -world is estimated at 
about 1,450 millions of individuals. About 1,200 millions, 
nearly 83% of mankind, are included in the Mediterranean 
and Mongolian races, and both of these races seem to be 
increasing in number and in intelligence, the increase and 
progress of the Mediterranean race being especially rapid. 
The other ten principal races of mankind are estimated 
to include at present less than 17% of the population of 
the world, and these races seem to be on the whole slowly 
decreasing in number and approaching extinction, as a 
direct or indirect result of the influence of the more intel- 
ligent and civilized Mediterranean race. The estimated 



364 



PHYSICAL GEOGRAPHY. 



number of individuals in each of the twelve principal races 
of mankind is given below : 



Mediterranean, 
Mongolian, . 
Negro, . . . 
Dravidian, 
Malay, . . . 
Kaffre, . . . 
American Indian, 
Nubian,. . . . 



625,000,000 
575,000,000 
130,000,000 
34,000,000 
30,000,000 
20,000,000 
12,000,000 
10,000,000 



Papuan, .... 
Australian, . . . 
Hottentot, . . . 
Esquimo, . . . 
Half-breeds of the 
various races 

Total, 



2,000,000 
100,000 
100,000 
100,000 

11,700,000 



1,450,000,000 



Man in the rudest state in which he now exists, is 
the most dominant creature that has ever appeared upon 
the earth. He forms the only highly organized species 
that has spread over the entire land surface of the globe. 
All other organisms have yielded before him. All savage 
men without exception seem to possess an articulate lan- 
guage, a knowledge of the art of making fire and of some 
of its uses, and the ability to make and use various rude 
weapons, tools, and traps with which to defend themselves 
and obtain food. Such inventions, by which the rudest 
savage achieves his pre-eminence among organisms, are 
the direct results of the development of his powers of 
observation, memory, curiosity, imagination, and reason. 

The reasons why certain tribes, and not others, have 
risen in the scale of civilization from this rude state can 
not be fully given. Progress seems to depend upon com- 
binations of favorable conditions far too complex to be 
followed out. The remarkable fact, however, has fre- 
quently been observed that all high civilization has devel- 
oped in the north temperate zone, and that the native 
races of this zone, when first visited by civilized men, had 
arrived at a higher state of civilization than the native 
tribes of the torrid and frigid zones. It would seem, 
therefore, that an extensive land area, combined with a 



MAN. 365 

temperate climate, is a physical condition favorable to the 
development of intelligence and civilization. 

Reflection suggests a possible explanation for this. Development 
is the result of mental activity, and hence would generally be 
greatest in a region that afforded the greatest incentive to mental 
action. An extensive region possesses a greater variety of environ- 
ments than a contracted region, and hence would develop a greater 
number of different tribes of men. The natural competition of 
these tribes for mastery, would form a constant incentive to greater 
mental activity in the larger region. In extensive temperate regions, 
an additional incentive to mental action is afforded by the effect of 
the climate upon vegetable and animal life, and hence upon man's 
food supply. The regular alternation in such regions of a long, 
warm summer, when food is plenty, with a long, cold winter, when 
food is very scarce, is in marked contrast to the constant summer 
of the torrid zone, which affords a perpetual abundance of food in 
the moist regions, but causes a perpetual dearth of food in the dry, 
desert regions ; while in the frigid zones there is a perpetual dearth 
of food because of the long, cold winters. Hence, the climate of 
the temperate zones is peculiar in affording a constant incentive to 
collect and cure a store of food during the summer upon which to 
draw during the following winter. The gradual development of fore- 
sight and ingenuity involved in such a collection and preservation 
of food for future use, would of itself raise a tribe in the scale of 
civilization, and such mental development would lead to further 
progress in other directions. 

Domestic Plants and Animals. — The great mass of 
mankind to-day — all, indeed, but the rudest tribes — de- 
pend chiefly for food and clothing upon domestic plants and 
animals. These form the portion of the organic world 
which man has subjugated. It is a remarkably small por- 
tion when compared with the world's flora and fauna. 
Among more than a million species of plants and animals, 
the cultivated plants form only about 300 species, and the 
domestic animals only about 200 species. By far the 
most important of these plants and animals were reduced 
to a domestic state in prehistoric times, and can not now be 



3 66 



PHYSICAL GEOGRAPHY. 



found in a truly wild state. That is to say, they have been 
under the peculiar environment caused by man's care, long 
enough to have varied in structure to such an extent, that 
the respective wild species, from which they originally 
sprung, can no longer be identified with certainty. 

The food plants, wheat, oats, barley, rye, rice, sugar-cane, tea; 
many garden vegetables, as the turnip, onion, cabbage, cucumber, 
watermelon, bean, and pea ; several fruits, as the European grape, 
mango, apricot, almond, peach, pear, apple, quince, pomegranate, 
olive, fig, date, and banana ; the fiber plants, cotton, flax, and 
hemp ; and the common domestic animals, the horse, ass, camel, and 
sheep, goats, cattle, and chickens, had been cultivated and domesti- 
cated by various races of Euro-Asia before the dawn of their re- 
spective histories, and in very early historic time coffee was 
cultivated. The native races of America, upon its discovery, though 
they knew nothing of the domestic plants and animals mentioned 
above, had domestic plants and animals of their own that were 
equally strange to the Europeans, such as the sweet-potato, the 
potato, maize or corn, the pumpkin, squash, tomato, mate or Para- 
guay tea, coca, cacao, the aloe, guava, pine-apple, peanut, tobacco, 
red pepper, and sea-island cotton, and the turkey, rabbit, guinea- 
pig, and llama. It is thus seen that the very plants and animals 
which America now supplies in such great quantities to the markets 
of the world, such as wheat, rice, sugar-cane, coffee, hemp, the 
common kind of cotton, and horses, cattle, sheep, swine, and 
chickens, are all of foreign origin, and were introduced by man 
since the discovery of the continent only 400 years ago ; while the 
potato and Indian corn, which are now largely raised in parts of 
Europe, were introduced from America, where they had previously 
been sparingly cultivated by the natives. 

Man's achievements over inorganic nature have in 
general been of much more recent date than those ovef 
the organic world. In fact, the progress of civilization 
during historic time is almost exclusively the result of 
discoveries and inventions by which inorganic substances 
can be applied to the uses of man, and this is specially 
true of the present age. 



MAN. ?>67 

Metals. — Among the inorganic substances whose use 
by man indicates his progress in civilization, are the 
metals. Few of these occur in a pure state in nature. 
Most of them are found as stony substances, or ores, in 
which the metal occurs only as a chemical ingredient, and 
from which it is obtained only by more or less intricate 
artificial processes. Some metals are found in the metallic 
state, but almost always alloyed, or mixed, with other 
metals, and their separation is generally difficult. But 
even after the pure metal is obtained, more or less difficult 
artificial processes are required to apply it to the various 
purposes of man. It is chiefly through an increasing 
knowledge of the laws of nature, by which the metals 
and other inorganic substances can be more easily ob- 
tained and applied, that modern civilization is advancing. 

The eight metals in most common use to-day — iron, copper, tin, 
zinc, lead, gold, silver, and mercury — seem to have been known in 
the earliest historic time ; but as geographical knowledge increased, 
the sources from which they could be obtained became more nu- 
merous, while, with the increase of physical knowledge, easier and 
cheaper methods for reducing them were discovered, and the ways 
in which they could be rendered beneficial to man multiplied pro- 
digiously. Not a day passes in which every individual of all civil- 
ized races does not repeatedly derive benefit, either directly or 
indirectly, from the use of most or all of these metals, and the 
amount and variety of their use by any people is an infallible index 
of their degree of civilization. 

Distribution. — Most of the metals or their ores are 
widely distributed over the globe, occurring among rocks 
of various geological ages. They are generally most 
abundant in regions of highly tilted, disturbed, or meta- 
morphosed strata, such as characterize mountain regions. 
This is partly due to the fact that the enormous erosion 
in such regions has exposed a greater variety of forma- 
tions ; but it seems probable that many of the metals were 

P. G.— 21. 



368 PHYSICAL GEOGRAPHY. 

forced up from below, in a state of solution, as a conse- 
quence of the upheaval of these regions, and, upon pre- 
cipitation in the fissures of the dislocated strata, combined 
with other substances present to form the ores and native 
alloys of the metalliferous veins and lodes. 

Iron, the most abundant and useful metal, is obtained from 
several kinds of ore — magnetic, hematite, limonite, etc. Iron ore 
is mined and reduced in every civilized country. Most of that re- 
duced in the United States, which yields about one fifth of the 
world's supply, comes from the south shore of Lake Superior, from 
the Appalachian region, and the Ozarks ; but great quantities of ore 
exist throughout the west. Copper is found as ore, and also in the 
metallic state. Copper is found in almost all regions of old meta- 
morphic rock. The richest mines in the world are in northern 
Michigan, and in Chile, Spain, and Australia. Tin, zinc, and lead 
are found as comparatively easily reducible ores. Tin is most exten- 
sively mined in the East Indies, Australia, and south-west England, 
but deposits are known to exist in Mexico, and in California, Alabama, 
and elsewhere in the United States. Zinc is mined in Illinois, Mis- 
souri, Kansas,^ New Jersey, Tennessee, and in several European 
countries, notably Germany and Belgium. Lead is largely produced 
in the Rocky Mountain region in the reduction of silver ore, but is 
mined in the Ozarks, Illinois, England, Germany, and Spain. Gold 
in the metallic state, occurs in veins of quartz in metamorphic rock, 
and as fine grains (gold dust) in stream deposits composed of the 
eroded and disintegrated debris of such rock. Traces of gold are 
found in such deposits in nearly all mountain regions, but the richest 
yet worked are in the Sierra Nevada of California, the Australian 
Alps, and the mountains of south-east Africa. The United States 
supplies about one third of the world's yield of gold. Silver is some- 
times found in a metallic state, but generally combined with sulphur 
as an ore. It is specially abundant in the Cordilleras of North and 
South America. Colorado affords about one half the silver product 
of the United States, which supplies about one half the yield of the 
world. Mercury melts at a temperature of about 37 below zero, 
and is the only metal that is liquid at ordinary temperatures. It 
usually occurs as an ore called cinnabar. Mines in the Coast Range 
of California supply about one half the world's annual yield. Most 
of the rest comes from the mountains east of the Adriatic Sea, and 



MAN. 369 

from Almaden, Spain. Antimony, platimim, and nickel are the only 
metals of comparatively recent discovery that have been largely used 
in the arts. The ore qf antimony is obtained chiefly in the East 
Indies, but is found in both Europe and North America. Platinum, 
like gold, is found in minute metallic grains in alluvial deposits. 
Three fourths of the world's supply comes from the Ural Mountains. 
Nickel ore in minute quantities is very widely distributed. The 
mines of the Sudbury district in Canada, furnish the world's chief 
supply. It is also found in Saxony, Sweden, and New Caledonia. 

Various other minerals besides the metals are largely 
collected and used by man. Among these are the many 
kinds of building stone, clays for making brick and 
pottery, marls for fertilizing the soil, salt, and the precious 
stones or gems, used both in the arts and for ornaments. 
But the mineral whose use is confined most exclusively to 
civilized man, and the loss of which would affect him 
most seriously, is coal, or mineral fuel. 

Coal is the most widely distributed and the cheapest 
source of great and easily available heat, or kinetic energy, 
that man has ever discovered. It is only since the recog- 
nition of its great thermal value, about 700 years ago, that 
iron and steel have been manufactured cheaply and in 
large quantities, while the rapid development of all kinds 
of manufacturing, which followed the invention of the 
steam engine 150 years ago, was largely due to the rela- 
tive abundance and cheapness of mineral fuel. 

Formation of Coal. — True coal, though a stony sub- 
stance occurring in layers interstratified with sedimentary 
rocks of various geological eras, is organic matter. It is 
chiefly the metamorphosed remains of a swamp vegetation 
which flourished on the earth's surface thousands of years 
ago. As such vegetation died and fell, it was covered 
with water, and thus protected from the atmosphere, and 
consequently from rapid decomposition into stable com- 
pounds — carbonic acid, etc. Thus, a thick layer of or- 



37° PHYSICAL GEOGRAPHY. 

ganic matter accumulated on the bottom of the swamp, 
which, when more deeply submerged by a gradual sub- 
sidence of the region, was covered and buried by layers 
of ordinary inorganic sediment. When subsequent eleva- 
tion, or the accumulation of sediment, brought the surface 
of the region nearly to the water surface again, swamp 
vegetation sprang up, and another incipient coal seam was 
formed, and so on. As the weight above increased, the 
buried layers of organic matter gradually became com- 
pacted, while the increase of temperature as they became 
more deeply buried (page 186) caused the complex and 
unstable protoplasmic compounds, rich in carbon, gradu- 
ally to break up and form simpler compounds in which 
carbon did not enter so largely, such as water, sulphuretted 
hydrogen, carbonic acid, marsh gas, etc. The residue 
was consequently left exceedingly rich in carbon, generally 
combined to a greater or less extent with hydrogen, as 
bitumen. This unstable residue constitutes coal, and the 
associated hydro-carbons, naphtha, natural gas, petroleum, 
mineral tar, asphaltum, etc. 

The carboniferous era of geological time marks the period when 
the bulk of the plant life of the globe had developed to a stage suit- 
able for the growth of swamp vegetation. In that era it seems to 
have consisted chiefly of gigantic ferns, cycads, and pines ; and 
such vegetation was very prevalent, causing by far the thickest and 
most extensive coal deposits yet discovered. Such coal-forming 
vegetable deposits occurred, however, during all subsequent ages, 
and are to-day accumulating, as peat, in very many localities, notably 
in the Dismal Swamp region of Virginia and North Carolina ; be- 
neath the swampy "trembling prairies" of southern Louisiana ; in the 
bogs of Ireland ; and about the shores of the Baltic. The transfor- 
mation of vegetable matter into true bituminous coal is an ex- 
cessively slow process. It seems in general to require as long a 
time as from the Jurassic period to the present, for as we go back 
among successively older rocks we find the peat deposits of the 
present era insensibly passing into lignite, or brown coal, in the 
tertiary era, which becomes more and more thoroughly carbonized 



MAN. 371 

as we proceed, until, in the Jurassic period, true bituminous coal 
occurs. In regions that have been subjected to exceptionally great 
heat or pressure the process has been hastened, and in some regions 
has progressed beyond the stage of bituminous coal ; thus, where the 
carboniferous strata in north-eastern Pennsylvania are most ex- 
tremely plicated and contorted, the inclosed seams of coal have lost 
much of their bitumen, and have been compressed into the hard, 
more thoroughly carbonized, and most valuable heating coal — an- 
thracite. In the older contorted rocks of Rhode Island and Mas- 
sachusetts, anthracite coal has advanced a stage further and lost 
much of its value as fuel, by transformation into nearly pure carbon 
ox graphite. In several localities in the West, where lava dikes have 
intersected the relatively young cretaceous and tertiary strata, the 
included seams of lignite are found to have been transformed, in 
the vicinity of the dikes, into true bituminous coal, or even an- 
thracite, by the great heat of the lava. 

Distribution. — Deposits of coal, near enough to the 
earth's surface to be accessible, are widely distributed. 
Fully one tenth of the area of the United States, and 
about the same proportion of Europe, are known to be 
underlaid by workable coal, while deposits of great but 
unknown extent exist in China, Canada, Australia, India, 
Chile, Brazil, and elsewhere. 

In the eastern part of the United States, coal of carboniferous age 
is found, under: (1) the whole Appalachian plateau from northern 
Pennsylvania to central Alabama — about 64,000 square miles; (2) 
the central part of southern Michigan — about 7,000 square miles ; 
(3) the southern two thirds of Illinois, south-western Indiana, and 
western Kentucky— about 47,000 square miles ; and (4) from central 
Iowa southward across western Missouri and Arkansas and eastern 
Nebraska, Kansas, and Indian Territory into central Texas — about 
99,000 square miles. The first of these coal-fields is by far the most 
extensively worked, and supplies more than three fourths of the 
yield of the United States, while practically all of our anthracite 
coal comes from the small area of this field in Pennsylvania. True 
bituminous coal of triassic age is found in central Virginia and 
North Carolina. In the western half of the United States the sur- 
face is largely composed of rocks more recent than the Jurassic, and 
the extensive coal deposits found in nearly all the territories have, in 



7,72 PHYSICAL GEOGRAPHY. 

general, advanced only to the stage of lignite. This, though valu- 
able as fuel, and closely resembling coal, is not so valuable for 
some manufacturing purposes. In the vicinity of dikes in this 
region, as before mentioned, and in the regions of contorted strata 
along the flanks of mountain ranges, where the great erosion has 
exposed older rocks, true bituminous and anthracite coal are found. 
Over 500 million tons of coal are used annually in the world. 
About one third of this is mined in the United States; over one 
half of the remainder in England ; and most of the rest in conti- 
nental Europe — Germany being by far the largest producer. 

Conclusion. — Thus, through their continued use, or 
exercise, man's mental powers have gradually increased. 
With his constantly expanding faculties, his observation 
of nature becomes more exact, and he daily recognizes 
more clearly the dependence of her manifold phenomena 
upon each other. In numerous instances he has gained a 
sufficiently clear comprehension of her great and immuta- 
ble laws to invoke their aid at will in securing results 
beneficial to himself. By incessant exertion, he maintains 
his limited power to thus direct the operation of these 
laws into such channels that they may produce about him 
the peculiar and artificial environment essential to civiliza- 
tion. But the observation of nature involved in the 
attainment of high civilization, results in far more than the 
development of man's inventive genius and the improve- 
ment of his material surroundings. In partially revealing 
the harmonious, yet marvelously intricate plan on which 
the world has been modeled, it teaches him of the utter 
insignificance of his own unaided powers, and increases his 
faith in and reverence for the Divine Wisdom which de- 
vised and which maintains it all. 



INDEX 



A 

PAGE 

Absorption of radiant energy 21 

Adaptation, of organisms 318 

effect of 327 

Adhesion 14 

Aerolites 38 

Affinity, chemical 9, 15 

Africa, characteristic life-forms of... 341 

surface of 174 

Age, of mountains 253 

of rocks 190 

of valleys 226 

Aggregation, states of 12 

Air, capacity for vapor 66 

composition of 55 

density of 57 

humidity of 67 

mechanical cooling of 69 

saturated 66 

weight of 56 

Alloys, metallic 367 

Amoeba 315 

Animals, classification of 321 

distribution of 328 

domestic 366 

how differ from plants 317 

vertebrate.. 323 

Antarctic regions, ice of no, 121 

land in 160 

size of. no 

Anticyclones, defined 95 

effect on weather 295 

Antitrade winds, defined 83 

Aqueous rocks 183 

Archipelago, Arctic 156 

Malay 156, 178 

Arctic Ocean, size, etc no 

Artesian wells 196 

Asia, life-forms of Oriental region. . . . 343 

monsoons of 86 



PAGE 

Asia, surface of 171 

Asphaltum 192, 370 

Athermanous bodies, defined 21 

Atlantic Ocean, bottom of 114 

coast-line of in 

depth of in 

drainage basin of 207 

size and shape of. 109 

temperature sections 118, 141 

winter winds of 88 

Atmosphere, color of 103 

defined 55 

density of 57 

distribution of vapor in 67 

electricity of. 106 

heat of 58 

height of 57 

moisture of 66 

pressure of 56, 8t 

uses of. 58, 59 

Atmospheric pressure at tropics, 

equator, and poles 82 

Atolls 159 

Atoms, defined 8 

Aurora Polaris 108 

Australia, characteristic life-forms of 339 

surface of 177 

Autumnal equinox, the 52 

B 

Bad Lands 257 

Barometer, the 56, 58 

Bayou 229 

Biological regions 335 

Brain of man and animals 35i~3 

Breakers 124 

Breathing, its effect 316 

Breeze, land and sea 89 

Buttes 259 

(373) 



374 



PHYSICAL GEOGRAPHY. 



C 

PAGE 

Calderas, how formed 284 

Calms, belts of 83 

Campos, pampas, etc 330 

Canons, formation of. 222 

in eastern United States 226 

of Colorado River 166, 222 

Capillary attraction 14 

Carbonic acid, in atmosphere 56 

in sea-water 116 

in spring- water 201 

Cataracts and cascades 224, 226 

Caverns, formation of 201 

Cells, differentiation of 316 

in living matter 314 

organisms composed of single 315 

segmentation of 315 

Centrifugal force 44 

Chemical affinity 15 

Chemical heat, how produced 19 

Circles, great 45 

small 46 

Civilization, ancient Egyptian 354 

development of 356, 365 

modern 367 

Cliffs, formation of 224, 258 

never very high 164 

Climate, continental and oceanic 301 

defined 297 

effect of elevation 308 

" " exposure 310 

" " land or water surface.. . .24, 299 

" " latitude 297 

" " mountain ranges 311 

" " ocean currents 304 

" " on life-forms 328 

in torrid zone 306 

on east and west coasts 302 

Cloud bursts 97 

Clouds, color of 104 

formation of. 69 

height of 69 

kinds of 70 

shape of 71 

uses of 71 

Coal „ 192, 369 

Cohesion, defined 9, n 

power of 13 

Color, of clouds and snow 104 

theory of 103 



PAGE 

Columnar structure of rocks 185 

Combustion 16 

Comets, described 37 

Compass, directions of the 44 

variation of the 32 

Compound substances 15 

Condensation, effect of 67 

Conduction, of electricity 33 

of heat 20 

Conservation of energy, defined 18 

Continental climate 301 

Continental islands, defined 155 

distribution of 156 

Continental plateau, described 151 

permanence of 148, 347 

Continents, characteristic life-forms of 338 

grand divisions of 155 

relative areas of 154 

Contraction and expansion of bodies 23, 25 

Convection of heat 20 

Coral islands and reefs 159 

Coronas 106 

Corrasion 216, 218 

Crater, volcanic 278, 284 

Crevasses, glacial 236 

in banks of streams 229 

Cryptogamic plants 320 

Crystallization, defined 12 

Currents, velocity in streams 213 

(see Ocean currents.) 

Cyclones, described 92 

effect on weather 293 

D 

Day and night, cause of 43 

length of 51, 298 

Decomposition 16 

Deltas 227 

Denudation 187 

Deposits, of geysers 286 

of lakes 244 

of sea 145 

of springs and percolating water. . . 202 

of streams 218, 227 

Deserts, defined 330 

Development, of cell 315 

of embryo 324 

of land life 348 

Development theory 326 

applied 333. 33 6 



INDEX. 



375 



PAGE 

Dew, definition of 74 

formation of 75 

Dew-point, defined 75 

Diathermanous bodies, defined 21 

Digestion, use of. 318 

Dikes, lava 185, 291 

Dip of rock strata 189 

Direction of the earth's rotation 44 

Distribution of life 346 

effect of climate on 328 

effect of isolation on 333 

marine 349 

Drainage basins, oceanic and inland. .206-7 

of stream systems 211 

Drift, glacial 236, 239, 240 

Dust, in atmosphere 56 

relation to mist or fog 69 

E 

Earth, the, axis of 43 

composition of crust 180 

condition of interior 42 

density of 41 

inclination of axis of 49 

internal temperature of. 41 

magnetism of 30, 32 

poles of 43 

position among planets 36 

orbit of 48 

radiates heat 59 

revolution of 48 

rotation of 43 

shape of 38 

size of. 39 

surface of. 149 

Earthquakes, causes of 190, 264, 272 

effects at earth's surface 272 

elastic waves of. 266 

energy of 271 

epicentrum of 269, 272 

frequency of. 265 

in United States 265, 276 

origin of 268 

propagation of 269, 271 

sea waves caused by 276 

sounds caused by 274 

Ecliptic, plane of 49 

Electrical condition of matter 32 

Electric spark, defined 34 

Electricity 28 



PAGE 

Electricity, atmospheric 106 

aurora 108 

generated in tornadoes 97 

generated in volcanic eruptions 285 

lightning 106 

St. Elmo's fire 108 

thunder 107 

Elements, chemical 7, 15 

in earth's crust 180 

Elevation, continental, region of 150 

decrease of temperature with 310 

mean, of land 161 

variation of life with 329 

Embryo, development of. 324 

Energy I? 

heat and light forms of. 18 

of earthquakes 271 

of volcanoes 285 

radiation of 19 

Environment, changes in 327 

defined 319 

Equator, defined 45 

the thermal 64 

Equinoxes, the 52 

Erosion, a cause of earthquakes 264 

amount of 219 

curve of. 220 

defined 181 

effect of corrasion upon 218 

effect on land surface 219, 256 

fantastic forms of 259 

in folded strata 259 

in mountain regions 249 

Estuaries 229 

Ether, luminiferous 19 

Euro-Asia, characteristic life-forms of 344 

surface of. 171 

Evaporation, cooling effect of 67 

defined 25 

from streams 208 

influences ocean currents 136, 144 

variation of 77 

Expansion and contraction of bodies 23, 25 

F 

Fata Morgana 102 

Faulted strata 189, 248, 252 

Fissures, caused by earthquakes 273 

Fiords 229 

Fixed stars 3*. 



376 



PHYSICAL GEOGRAPHY. 



PAGE 

Floods in streams 213, 215 

Fog, explained 68 

Food, use of 317 

Forces of nature 9 

Forest regions 330 

Fossils 183, 191 

order of appearance 325 

Fragmental rocks 183 

Frost, hoar -. 75 

on hillsides 311 

Fumaroles 280 

G 

Gas, defined 12 

Geological time, length of 193 

Germ-cell 314 

development of 315 

Geysers 286 

Glaciers, distribution of. 231, 238 

effects on the land 236, 239 

formation of 231 

former extent of 237 

melting of 234 

moraines of 235 

movements of 233 

size of 233 

streams from 237 

Grand divisions of land 155 

average elevation of 161 

Graphite 371 

Gravitation, defined 9 

effect of distance on 10 

some effects of 11 

Great Lakes, the formation of. 240 

Gulf Stream, the 140, 142 

H 

Hail, formation of 74 

produced by tornadoes 97 

Halos 106 

Heat, affects size of bodies 22 

all bodies possess 19 

conduction and convection of 20 

conveyed by ocean currents 61, 140 

how imparted to atmosphere 58 

latent 24, 61 

mechanical equivalent of 26 

nature of 18 

radiation of 19 

reflection, transmission, and absorp- 
tion of 21 



TAGE 

Heat, specific 23,60 

Hemispheres 45 

Heredity, defined 318 

effect of 327 

Highlands, defined 161 

of Africa 174 

of Euro-Asia 171 

of North America 166 

of South America 169 

of world 154, 179 

Hills, defined 162 

Hoar-frost 75 

Horizon, defined 39 

sun appears large near 100 

sun visible below 100 

Humidity 67 

Hurricanes 94 

Hygrometer 68 

I 

Icebergs ZI g t 142 

Ice, crystals 13, 72 

formation of 25 

of the sea 118 

Igneous rocks 185 

Impenetrability of matter 8 

Indestructibility of matter 8 

Indian Ocean, bottom of 114 

coast-line of 111 

currents of north 140 

depth of : in 

drainage basin of 207 

size and shape of 109 

Induction, electrical 34 

magnetic 29 

Inertia (centrifugal force) 44 

defined 8 

Intus-susception 314 

Islands, continental 156 

coral 159 

life-forms of oceanic 336 

oceanic 157 

Isotherms, defined 61 

of United States 309 

of world 63, 65 

K 

Kinetic energy 17 

Kuro Si wo, the 140 

L 

Laccolites 283 

Lagoons 247 



INDEX. 



377 



PAGE 

Lakes, color of 245 

crescent-shaped 230 

distribution of 246 

effect on climate 304 

effect on floods 245 

formation of 241 

fresh and salt water 242 

obliteration of 244 

shape of 246 

spring 201 

temperature of 245 

the Great, of United States 240 

Lambert's projection 53 

Land, area of 149 

effect of erosion on 219, 256 

effect of glaciers on 239 

effect on climate 60, 64, 299 

has been submerged 188 

height of 112, 150, 161 

slips or slides 204 

surface of. 1 ( >I 

Languages, resemblances of 357 

Latent heat 24 

amount of 61 

Latitude, defined 46, 47 

effect on climate 297 

variation of life with 328 

Lava, composition of. 180 

dikes 185, 282 

fluidity of 280 

how discharged 279 

laccolites 283 

streams 280 

Level of the sea 149 

Life, ancient fossil 325 

classification of 319 

distribution of land- 328, 331, 346 

forms compared 324 

great regions of 335, 346 

higher forms of 315 

manifestation in matter 313 

marine 349 

simplest forms of. 315 

Light, diffusion of 27 

nature of 18 

phenomena of 100 

refraction of 26 

selective absorption of 28 

Lightning 106 

Lignite 371 



PAGE 

Liquid, defined 12 

Living matter, peculiarities of 314 

Llanos, pampas, etc 330 

Longitude, explained 46, 47 

Looming 102 

Lowlands, defined 161 

of Africa 177 

of Euro-Asia 172 

of North America 169 

of South America 170 

of world 179 

Luminous bodies 19 

phenomena 100 

M 

Magnetic storms 32 

Magnetism, cause of earth's 32 

described 28 

of earth 30 

Man, ancestral race of. 356, 358 

ancestry of 353 

compared with animals 350, 353 

in savage state 364 

prehistoric 354, 355 

races of 358, 364 

Map projections 53 

Marsupial animals 323 

Mass, explained 9 

Matter, defined 7 

organic 314 

properties of 8 

Meadows, life-forms of 330 

Mechanical cooling and heating of air 69 

Mechanical equivalent of heat 26 

Mercator's projection 53 

Meridians, defined 45 

Mesas 257 

Metalloids 15 

Metals 15, 367 

distribution of 367 

Metamorp rocks 185, 186 

Meteors 37, 58 

Mi lcral , *ined 180 

dissolved in lake and river water. . 242 

" in sea-water 115 

" in spring-water 200 

of great use to man 367 

Mirage 102 

Mist, explained 68 

Molecular motion 18 



37% 



PHYSICAL GEOGRAPHY. 



PAGE 

Molecules, defined 8 

motion of . . . . 19 

Monotreme animals 323 

Monsoons, defined 84 

effect of in Indian Ocean 140 

of Asia and Australia 86 

of North America 88 

Moon 37 

cause of tides 130 

Moraines, defined 235 

in United States 239 

Mountains, age of 253 

defined 162 

erosion of 249, 259 

granitic crests of 263 

influence on climate 311 

influence on winds 89 

of Africa and Australia 176, 178 

of America 166, 170 

of Euro-Asia 171 

of faulted strata 252 

of folded strata 249, 261 

origin of 255 

rate of upheaval 253 

stratified rock in 255 

structure of 248 

table, or mesas 257 

Movements of earth's crust 148, 190 

cause earthquakes 264 

effect distribution of life 338, 347 

fold and break strata 188 

in mountain regions 253 

N 

Natural gas 192, 370 

Nature, forces of 9 

laws of 7 

Nebular theory, the 38 

Newfoundland Banks 142 

Niagara, cataract of 225, 226 

Night, cause of 43 

length of 51, 298 

Nitrogen, in air 55 

in sea-water 116 

North America, characteristic life- 
forms of 344 

surface of 166 

Nutrition 317 

O 

Ocean currents, causes of 135 

deep 142 



PAGE 

Ocean currents, direction of 136 

effect on climate 61, 304 

" " sea temperature 140,143 

Gulf Stream 140 

into land-locked seas 144 

surface 137 

Oceanic climate 301 

drainage basins 207 

Oceanic islands 157 

life-forms of 336 

Oceans, boundaries and dimensions.. 109 

Oozes, on sea-bottom 146 

Orbit, defined 36 

of earth 48 

of moon 133 

Ores, metallic 367 

Organic matter, defined 314 

Organisms, cause of variety of 327 

classification of 319 

defined 314 

increasing complexity of 324 

individuality of 318, 319 

order of appearance on earth 325 

Organs, defined 314 

development of 316 

Oxidation 16 

Oxygen 15 

in air 55 

in life-processes 3 1 ^~7 

in sea-water 116 

P 

Pacific Ocean, bottom of 114 

coast-line of in 

depth of in 

drainage basin of 207 

size and shape of 109 

temperature sections of 118, 141 

tides of 133 

winter winds of 88 

Pampas, steppes, etc 330 

Parallels 45 

Peat 37° 

Petrifactions 192 

Petroleum 192, 370 

Phenogamic plants 320 

Physical Geography, defined 7 

Plains, defined 162 

rock strata in 248 

Planetoids 37 



INDEX. 



379 



PAGE 

Planets , 35, 36 

origin of 38 

Plants, classification of 320 

cultivated 366 

differ from animals 317 

distribution of 328 

how they feed 317 

Plateau, continental 151 

defined 162 

formation of 257 

submarine 114 

Polar circles and tropics 50 

Polar light, the 108 

Polar projection 54 

Pole, north and south 44 

Pole star 45 

Population of world 364 

Port, establishment of the 132 

Potential energy 17 

Prairies, steppes, etc 330 

Precipices, never high 164 

Precipitation, distribution and amount 75 
(see Rain-fall). 

Prehistoric man 354~5 

Pressure, atmospheric 56 

belts of high 82 

Probabilities, weather 294-297 

Projections 53 

Proteids, defined 313 

Protococcus 315 

Protoplasm, composition of 313 

how made 317 

in germ-cell 314 

motions in 315 

R 

Races, of men 358 

tidal 129 

Radiation of energy 19 

Rainbow, the 105 

Rain-fall, defined 75 

effect of ocean currents on 306 

effect on vegetation 311, 330 

influence of mountains on 312 

in United States 303, 307 

on land surface 77 

proportion discharged by streams.. 208 

winter and summer 302 

Rain, formation of 71 

in cyclones 94, 295 



PAGE 

Rain, in equatorial calms 136 

in tornadoes 97 

uses of 72 

Rain-water 71 

Rapids 224, 226 

Reefs, coral 159 

Reflection, of radiant energy 21 

total 27 

Refraction, defined 26 

displacement of sun and moon by. 100 

Mirage, Looming, etc 102 

Relative humidity 67 

Respiration 316 

Revolution of the earth 48 

Rivers, of Great Plains 224 

relative size of 210 

(see Streams). 

Rocks, classification by age 193 

columnar structure of 185 

composition of 180 

corrasion of 216 

disintegration of 181 

effect of glaciers on 236 

erratic 237 

metamorphic 185 

permeability of 195 

plastic under pressure 42, 151, 164 

position in mountains 249, 254, 260 

primitive 187 

solution of 181 

stratified 183 

un stratified 184 

weathering of 181 

Rock-tables 235 

Rotation of the earth 43 

influences currents 136 

influences winds 79 

S 

Saint Elmo's fire 108 

Salt-water lakes 242 

Salt water of sea 114, 243 

Sand-bars in streams 230 

Saturated air, defined 66 

Sea, bottom of 114, 145, 148 

continuity of no 

currents, affect temperature 140, 143 

" causes of 135 

" deep. 142 

" to arms of 144 



3 8o 



PHYSICAL GEOGRAPHY. 



PAGE 

Sea, currents, surface 137 

deposits of 145 

depth of in, 112 

earthquake waves of 276 

extent of 109 

ice of 118 

level of the 149 

saltness of 114, 116, 243 

temperature of 117, 140, 143 

tides of 125 

waves of 122 

Seasons, the 52 

wet and dry 306 

Sea-water, composition of. 114, 243 

Sedimentary rocks, formation of 183 

in mountain regions 255 

Sediment, forms rocks 183 

in glacial streams 237 

in lakes 244 

in streams 218 

Selective absorption 28, 104 

Sierra, definition of 263 

Simooms 93 

Simple substances 7, 15 

Sink-holes 201 

Sky, color of 103 

Sleet, origin of 72 

Slopes, of stream-beds 212, 220 

of valley sides 221 

influence on climate 310 

steepness of. 163 

Snow, color of 104 

formation of. 72 

uses of 74 

Snow-flakes, shape of 72 

Snow-line 73, 310 

Snow-storms 73 

Soil, defined 181 

Solar spectrum 28 

Solar system 35 

origin of 38 

Solfatara 286 

Solid, defined 12 

Solstices 52 

Solution of rocky matter 181 

in lakes 242 

in sea 114, 243 

in springs 200, 202 

in streams 216, 219 

Sounds, produced by earthquakes. . . 274 



PAGE 

South America, characteristic life- 
forms of 340 

surface of. 169 

Specific gravity 10 

Specific heat, explained 23 

of land and water surfaces 60 

Specter of the Brocken 102 

Spectrum, the solar 28 

Speed, of earthquake propagation... 269 

of earth's revolution 48 

of earth's rotation 44 

of light 20 

of lightning 107 

of stream currents 213 

of tidal waves 127 

of water waves 123 

Spheroid 39, 44 

Spring lakes 201 

Springs, deep seated 196 

deposits of. 202 

effect of earthquakes on £73' 

hot 198, 286 

intermittent 196 

mineral 200 

surface 195 

temperature of 198 

uses of 198 

Stalactite 203 

Stalagmite 203 

Stars, fixed 35 

Steam, in geysers 286 

in volcanoes 279, 292 

latent heat of 25 

Steppes, prairies, etc 330 

Storms, magnetic 32 

paths of 91 

wind 90 

zones of 94 

Stratified rocks 183 

disturbed and faulted 188 

position in mountain regions. .. .249, 254 

relative age of 192 

unconformable 190 

Streams, alluvial bottoms of 227 

cataracts and cascades of 224 

channels of 230 

corrasion of bed 216 

defined 205 

deltas of 227 

Streams, discharge of 208, 210 



INDEX. 



38l 



PAGE 

Streams, floods of 214, 245 

glacial 237 

great age of 226 

material transported by 219 

rapids 226 

sinuosities in course 229 

size of 210 

slope of. 212 

speed of 213 

system of , , 206 

valleys formed by 219 

volume of. 213 

Striae 236 

Sun 36 

distance of 48 

effect on streams 78 

effect on tides 130, 131 

effect on vegetation 317 

effect on winds 78 

heat, imparted to atmosphere 58 

heat, varies with latitude 49, 297 

source of heat (energy) 58 

" Sun drawing up water " 102 

Sunsets, red, of 1883 104 

Surface, movements of sea- 122 

of Africa 174 

of Australia 177 

of earth 149 

of Euro-Asia 171 

of land 161 

of North America 166 

of sea-bottom 144 

of South America 169 

Suspension, rocky matter in.. 217, 219, 245 



Talus 164, 224 

Temperate zones, weather in 294 

Temperature, abnormal 305 

annual range of 298 

decreases with elevation 59, 310 

effect of currents on sea- 140 

increases with elevation 311 

mean annual in United States 309 

measure of 22 

of atmosphere 59, 310 

of deep sea 117, 118, 142 

of earth's interior 41 

of lakes 245 

of land and water 60, 64 



PAGE 

Temperature, of northern hemisphere 61 

of sea-surface 117, 118 

of southern hemisphere 62 

of springs 198 

of valleys in winter 311 

relation to specific heat 23 

Tenacity of various substances 13 

Terraces of glacial drift 240 

Thermal equator, the 64 

Thermometer, described 22 

wet and dry bulb 68 

Thunder 107 

Thunder-storms 97 

Tidal currents 126, 227 

waves 127 

Tides 125 

cause of 130 

diurnal inequality of 133 

duration of 129 

" establishment of the port '' 132 

height of 128 

in lakes and seas 134 

on Pacific and Gulf coasts 133 

races caused by 129 

spring and neap 132 

the " bore " 129 

Tornadoes 96 

frequency in United States 99 

Torrid zone, weather in 293 

Trade winds, the 83 

Transparent bodies, defined 21 

Transportation of sediment 216, 219 

Tropical belts of high pressure 82 

Tropics and polar circles 50 

Tundras 329 

Twilight 101 

Typhoons 94 

U 

Unconformable strata 190 

Unstratified rock 184, 185 

V 

Valleys 163 

age of 226 

buried in drift 240 

canoe-shaped 261 

formation of. . . , 219, 224 

frosts in 311 



3 82 



PHYSICAL GEOGRAPHY. 



PAGE 

Valleys, glaciated 236 

sides of 221 

Vapor, amount in atmosphere 55 

described 66 

distribution in atmosphere 67, 77 

humidity of air 67 

Variation of the compass 32 

Vegetation regions 331, 332 

Velocity (see Speed). 

Vernal equinox, the 52 

Vertebrate animals 323 

Volcanic necks 291 

Volcanoes, activity of 278, 286 

calderas of 284 

causes of action 291 

cones of 281 

craters of 278, 284 

distribution of 179, 288 

eruptions of 40, 278, 284 

materials discharged from 279 

mud..... 287 

W 

Water, freezing of 13, 25 

latent heat of 25 

maximum density of 25 

movements in waves 122, 126 

specific heat of 23, 26 

surface affects climate 24, 60, 64, 299 

Water-gaps 226, 262 

Water-shed 206 

Water-spouts 99 

Waves 122, 127 

elastic 266 

force of 124 

from submarine earthquakes 276 

size and speed of 123, 127 

tidal waves 127 



PAGE 

Weather 293 

effect on rock 181, 222 

Weight, defined 9 

of air 56 

Wells, artesian 196 

surface 195 

White-caps 124 

White squalls 99 

Winds 78 

anticyclones 95 

antitrade 83 

cause of 78 

classes of 81 

cyclones 93 

cyclonic or storm 90 

diurnal, in valleys : . . . . 89 

dust whirlwinds 92 

effect earth's rotation on the 79 

effect on ocean currents 137 

force or speed of 81, 90 

hurricanes. 94 

land and sea breezes 89 

monsoons 84 

of oceans in winter 88 

of world 85, 87 

simoons 93 

spiral whirl of. 80 

tornadoes 96 

trade 83 

typhoons 94 

white squalls 99 

Winter solstice 52 

Y 

Yeast 315, 320 

Z 

Zones of the earth 49 



IIP 




